The libvirt config of my vga passthrough vm

libvirt-quadro.xml

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
    <name>Windows8-OVMF1</name>
    <uuid>d3d9d84e-3ce4-d77e-9bcc-d1fb5ba85813</uuid>
    <memory unit='GiB'>8</memory>
    <memoryBacking>
        <hugepages/>
        <locked/>
        <nosharepages/>
    </memoryBacking>
    <memtune>
        <hard_limit unit="G">10</hard_limit>
    </memtune>
    <cpu mode='host-passthrough' match='exact'>
        <topology sockets='1' cores='4' threads='2'/>
        <!--<feature policy='require' name='invtsc'/>-->
    </cpu>
    <vcpu placement='static'>8</vcpu>
    <cputune>
        <vcpupin vcpu='0' cpuset='6'/>
        <vcpupin vcpu='1' cpuset='7'/>
        <vcpupin vcpu='2' cpuset='4'/>
        <vcpupin vcpu='3' cpuset='5'/>
        <vcpupin vcpu='4' cpuset='2'/>
        <vcpupin vcpu='5' cpuset='3'/>
        <vcpupin vcpu='6' cpuset='0'/>
        <vcpupin vcpu='7' cpuset='1'/>
    </cputune>
    <os>
        <type arch='x86_64' machine='pc'>hvm</type>
        <loader readonly='yes' type='pflash'>/usr/share/edk2.git/ovmf-x64/OVMF_CODE-pure-efi.fd</loader>
        <nvram template='/usr/share/edk2.git/ovmf-x64/OVMF_VARS-pure-efi.fd' />
        <bootmenu enable='yes' timeout='3000'/>
    </os>
    <features>
        <acpi/>
        <apic/>
        <hap/>
        <hyperv>
            <relaxed state='on'/>
            <vapic state='on'/>
            <spinlocks state='on' retries='8191'/>
        </hyperv>
        <kvm>
            <hidden state='off'/>
        </kvm>
    </features>
    
    <clock offset='localtime'>
        <timer name='rtc' tickpolicy='catchup'/>
        <timer name='pit' tickpolicy='delay'/>
        <timer name='hpet' present='no'/>
        <timer name='hypervclock' present='yes'/>
    </clock>
    <on_poweroff>destroy</on_poweroff>
    <on_reboot>restart</on_reboot>
    <on_crash>restart</on_crash>
    <pm>
        <suspend-to-mem enabled='no'/>
        <suspend-to-disk enabled='no'/>
    </pm>
    <devices>
        <emulator>/usr/local/bin/qemu-system-x86_64_quadro</emulator>
        
        <controller type='usb' index='0' model='nec-xhci' />
        <controller type='scsi' index='0' model='virtio-scsi'/>

        <disk type='file' device='cdrom'>
            <source file='/home/vm/de_windows_8.1_n_with_update_x64_dvd_4048300.iso'/>
            <target dev='vdc' bus='scsi'/>
            <address type='drive' controller='0' bus='0' target='1' unit='0'/>
            <readonly/>
            <boot order="2"/>
        </disk>

        <disk type='file' device='cdrom'>
            <source file='/home/vm/virtio-win-0.1-81.iso'/>
            <target dev='hdd' bus='ide'/>
            <readonly/>
        </disk>

        <disk type='block' device='disk'>
            <driver name='qemu' type='raw' cache='none' io='native'/>
            <source dev='/dev/sdc'/>
            <target dev='vda' bus='virtio'/>
            <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
            <boot order="1"/>
        </disk>
        
        <interface type='bridge'>
            <mac address='52:54:06:3c:28:0b'/>
            <source bridge='br0'/>
            <model type='virtio'/>
        </interface>
        
        <hostdev mode='subsystem' type='usb'>
            <source>
                <vendor id='0x17ef'/>
                <product id='0x6009'/>
            </source>
        </hostdev>
        <hostdev mode='subsystem' type='usb'>
            <source>
                <vendor id='0x046d'/>
                <product id='0xc50e'/>
            </source>
        </hostdev>

        <hostdev mode='subsystem' type='pci' managed='yes'>
            <source>
                <address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
            </source>
            <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0' multifunction='on'/>
        </hostdev>
        <hostdev mode='subsystem' type='pci' managed='yes'>
            <source>
                <address domain='0x0000' bus='0x01' slot='0x00' function='0x1'/>
            </source>
            <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x1'/>
        </hostdev>
        
        <memballoon model='virtio'>
            <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
        </memballoon>

        <input type='keyboard' bus='usb'/>
        <input type='tablet' bus='usb'/>
        <graphics type='vnc' port='5901' autoport='no' listen='0.0.0.0' keymap='de'/>
        <video>
            <model type='qxl'/>
        </video>
    </devices>
</domain>

hello,i create my guest with this xml. my guest can running ,but my dislplay is not light ,why?