virt-host-validate-common: Print warning on missing IOMMU

virt-host-validate-common: Print warning on missing IOMMU

No only coverity warns about this, but it kind of makes sense
too. We have a test whether host supports IOMMU. Some platforms
don't have it, I know. But in that case we should print a message
that it's unknown whether platform has it or not.

Before:
  (no output)

After:
  QEMU: Checking for device assignment IOMMU support                         : WARN (Unknown if this platform has IOMMU support)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>