Add iommu device when VM configured with > 255 vcpus

[Jim Fehlig via Devel]

Hi All,

I vaguely recall a discussion about $subject, but can't find it now. Perhaps 
buried in another thread. The topic has been raised internally again, and I'd 
like to gauge the community's interest in automatically adding the necessary 
devices/config when user has specified vcpus > 255.

The comparison for prior art is a bit of a stretch, but we e.g. add <audio 
type='spice'/> when spice graphics is configured. I know libvirt has generally 
tried to avoid policy decisions, but it's not clear to me where we stand with 
cases such as this, where every x86 VM with > 255 vcpus needs a similarly 
configured iommu.

Regards,
Jim