x86-iommu: switch intr_supported to OnOffAuto type

x86-iommu: switch intr_supported to OnOffAuto type

Switch the intr_supported variable from a boolean to OnOffAuto type so
that we can know whether the user specified it or not.  With that
we'll have a chance to help the user to choose more wisely where
possible.  Introduce x86_iommu_ir_supported() to mask these changes.

No functional change at all.

Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>