x86_iommu: Move machine check to x86_iommu_realize()

Instead of having the same error checks in vtd_realize()
and amdvi_realize(), move that over to the generic
x86_iommu_realize().

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
