[libvirt] Missing CPU vendor in capabilities in 1.2.18+

[Guido Günther]

Hi,
We have a report in Debian[1] where after a update from 1.2.16 to 1.2.18
VMs on a Intel i5 CPU fail to start due to libvirt not being able to
match the CPU vendor:

    # virsh start sid
    error: Failed to start domain sid
    error: unsupported configuration: guest and host CPU are not compatible: host CPU vendor does not match required CPU vendor Intel

The root cause seems to be that the <vendor> element is missing from the
hosts capabilities. While I have:

    $ virsh capabilities | xpath -q -e //host/cpu/vendor 
    <vendor>Intel</vendor>

on my Intel box the reporters machine with a Core i5 the <vendor> element is just
empty. Any idea what could have triggered this? This is still broken in
1.2.19-rc1. The bug has more details and logs attached. Any pointers
would be welcome since I don't have access to such a machine atm to step
through src/cpu/cpu_x86.c.

Cheers,
 -- Guido


[1] http://bugs.debian.org/797513