Interesting, your SandyBridge machine is likely missing x2apic feature. If had that feature, libvirt would report SandyBridge instead of Westmere + avx. Could you try changing the XML to contain
<cpu mode='custom' match='exact'> <model fallback='forbid'>SandyBridge</model> <feature policy='disable' name='x2apic'/> </cpu>
and see if that makes any difference?
Interesting, your SandyBridge machine is likely missing x2apic feature. If had that feature, libvirt would report SandyBridge instead of Westmere + avx. Could you try changing the XML to contain
<cpu mode='custom' match='exact'> 'forbid' >SandyBridge< /model>
<model fallback=
<feature policy='disable' name='x2apic'/>
</cpu>
and see if that makes any difference?