Description of problem:
Guest has three video devices, and the vram or ram size of the second video and the third video are not equal. Start the guest, and migration will fail:
# virsh migrate rhel7.2 qemu+ssh://hp-dl385g7-06.lab.eng.pek2.redhat.com/system --live --verbose
error: internal error: qemu unexpectedly closed the monitor: 2016-07-20T09:17:43.220520Z qemu-kvm: Length mismatch: 0000:00:09.0/qxl.vram: 0x2000000 in != 0x4000000: Invalid argument
2016-07-20T09:17:43.221314Z qemu-kvm: error while loading state for instance 0x0 of device 'ram'
2016-07-20T09:17:43.222751Z qemu-kvm: load of migration failed: Invalid argument

Version-Release number of selected component:
libvirt-2.0.0-2.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Prepare a guest with three videos, and set different ram or vram size for the second video and the third video:
# virsh dumpxml rhel7.2 --inactive
...
    <video>
      <model type='qxl' ram='1048576' vram='16384' vgamem='16384' heads='1' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <video>
      <model type='qxl' ram='65536' vram='**32768**' vgamem='8192' heads='3'/>  
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
    </video>
    <video>
      <model type='qxl' ram='65536' vram='**65536**' vgamem='8192' heads='7'/>  
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
    </video>
...

2.Start guest
# virsh start rhel7.2

3.Check live vram/ram size, find that the value of the second video devices becomes 65536:
# virsh dumpxml rhel7.2
...
    <video>
      <model type='qxl' ram='524288' vram='16384' vgamem='16384' primary='yes'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <video>
      <model type='qxl' ram='65536' vram='65536' vgamem='8192'/>
      <alias name='video1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
    </video>
    <video>
      <model type='qxl' ram='65536' vram='65536' vgamem='8192'/>
      <alias name='video2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
    </video>
...

4.Do migration and failed.

Actual results:
Migration failed

Expected results:
Migration can succeed

Additional info:
1)
The qemu command line on source host:
-device qxl-vga,id=video0,ram_size=1073741824,vram_size=16777216,vram64_size_mb=0,vgamem_mb=16,bus=pci.0,addr=0x2 -device qxl,id=video1,ram_size=67108864,vram_size=33554432,vram64_size_mb=0,vgamem_mb=8,bus=pci.0,addr=0x9 -device qxl,id=video2,ram_size=67108864,vram_size=67108864,vram64_size_mb=0,vgamem_mb=8,bus=pci.0,addr=0xb

2)
The qemu command line on target host:
-device qxl-vga,id=video0,ram_size=536870912,vram_size=16777216,vram64_size_mb=0,vgamem_mb=16,bus=pci.0,addr=0x2 -device qxl,id=video1,ram_size=67108864,vram_size=67108864,vram64_size_mb=0,vgamem_mb=8,bus=pci.0,addr=0x9 -device qxl,id=video2,ram_size=67108864,vram_size=67108864,vram64_size_mb=0,vgamem_mb=8,bus=pci.0,addr=0xb

3)
I tried several times, and found that the vram/ram size of the second video device is always adjusted to the vram/ram value of the third video device