qemu: Enable I/O APIC if needed

qemu: Enable I/O APIC if needed

This is a follow up of my previous commits. If the number of
vCPUs exceeds some arbitrary value (255) then QEMU requires IOMMU
with EIM and intremap enabled. But in turn, intremap IOMMU
requires split I/O APIC (per virDomainDefIOMMUValidate()). Since
after my previous commits (e.g. v10.10.0-rc1~183) IOMMU is added
automagically, the I/O APIC can be also enabled automagically.

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