x86_iommu/amd: Enable Guest virtual APIC support

x86_iommu/amd: Enable Guest virtual APIC support

Now that amd-iommu support interrupt remapping, enable the GASup in IVRS
table and GASup in extended feature register to indicate that IOMMU
support guest virtual APIC mode. GASup provides option to guest OS to
make use of 128-bit IRTE.

Note that the GAMSup is set to zero to indicate that amd-iommu does not
support guest virtual APIC mode (aka AVIC) which would be used for the
nested VMs.

See Table 21 from IOMMU spec for interrupt virtualization controls

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: Tom Lendacky <Thomas.Lendacky@amd.com>
Cc: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>