qemu_domain: Automagically add IOMMU if needed

qemu_domain: Automagically add IOMMU if needed

If a Q35 domain has huge number of vCPUS (over 255, currently), then
it needs IOMMU with Extended Interrupt Mode enabled (see check in
qemuValidateDomainVCpuTopology()).

Well, we already add some devices and to other tricks when
parsing new domain XML. Might as well add IOMMU device if above
condition is met.

Resolves: https://issues.redhat.com/browse/RHEL-65844
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>