q35: Allow only supported dynamic sysbus devices

q35: Allow only supported dynamic sysbus devices

The only user-creatable sysbus devices in qemu-system-x86_64 are
amd-iommu, intel-iommu, and xen-backend.  xen-backend is handled
by xen_set_dynamic_sysbus(), so we only need to add amd-iommu and
intel-iommu.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20171125151610.20547-7-ehabkost@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>