KVM /sys/class/net/eth0/speed not readable

speed_unreadable

# stat /sys/class/net/eth0/speed 
  File: `/sys/class/net/eth0/speed'
  Size: 4096            Blocks: 0          IO Block: 4096   regular file
Device: dh/13d  Inode: 6340        Links: 1
Access: (0444/-r--r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2014-10-28 15:27:49.719631816 +0100
Modify: 2014-10-28 15:27:49.719631816 +0100
Change: 2014-10-28 15:27:49.719631816 +0100
 Birth: -
# cat /sys/class/net/eth0/speed
cat: /sys/class/net/eth0/speed: Invalid argument
# ll /sys/class/net/eth0/speed
-r--r--r-- 1 root root 4,0K Okt 28 15:27 /sys/class/net/eth0/speed
# more /sys/class/net/eth0/speed
# echo $?
0
# tail /sys/class/net/eth0/speed
tail: error reading `/sys/class/net/eth0/speed': Invalid argument