A very rough sample that demonstrates mounting a disk on a VM.

mount_a_disk.py

        si = connect.SmartConnect(host='vcsa',
                                  user='my_user',
                                  pwd='my_password')
        content = si.RetrieveContent()

        datacenters = [entity for entity in content.rootFolder.childEntity
                        if hasattr(entity, 'vmFolder')]

        dc = None
        for datacenter in datacenters:
            if datacenter.name == 'MyDatacenter':
                dc = datacenter

        search = content.searchIndex
        vm = search.FindByDatastorePath(dc, '[MyStorage] virtual_machine/virtual_machine.vmx')

        # the details we will need to make the disk:
        disk_path = '[storage0] disks/some_disk_file.vmdk'
        capacity = 16 * 1024 * 1024

        controller = None
        devices = vm.config.hardware.device
        for device in devices:
            if 'SCSI' in device.deviceInfo.label:
                controller = device

        virtual_disk = vim.vm.device.VirtualDisk()

        # https://github.com/vmware/pyvmomi/blob/master
        # /docs/vim/vm/device/VirtualDisk/FlatVer2BackingInfo.rst
        virtual_disk.backing = \
            vim.vm.device.VirtualDisk.FlatVer2BackingInfo()

        # https://github.com/vmware/pyvmomi/blob/master
        # /docs/vim/vm/device/VirtualDiskOption/
        # DiskMode.rst#independent_persistent
        virtual_disk.backing.diskMode = \
            vim.vm.device.VirtualDiskOption.DiskMode.persistent

        virtual_disk.backing.thinProvisioned = False
        virtual_disk.backing.eagerlyScrub = True
        virtual_disk.backing.fileName = disk_path

        virtual_disk.connectable = vim.vm.device.VirtualDevice.ConnectInfo()
        virtual_disk.connectable.startConnected = True
        virtual_disk.connectable.allowGuestControl = False
        virtual_disk.connectable.connected = True

        virtual_disk.key = -100
        virtual_disk.controllerKey = controller.key
        virtual_disk.unitNumber = len(controller.device)
        virtual_disk.capacityInKB = capacity

        device_spec = vim.vm.device.VirtualDiskSpec()
        device_spec.operation = vim.vm.device.VirtualDeviceSpec.Operation.add
        device_spec.device = virtual_disk

        spec = vim.vm.ConfigSpec()
        spec.deviceChange = [device_spec]

        task = vm.ReconfigVM_Task(spec)
        while task.info.state in [vim.TaskInfo.State.queued,
                                  vim.TaskInfo.State.running]:
            print(".")
        print("done.")

a suggestion from an off line review:

use something like:
if isinstance(device, vim.vm.device.VirtualSCSIController)
it might be more reliable in case we have some other device with SCSI in the label that isn't a controller.

Anyone seriously looking to use this script in a production environment should consider that.