vdpa: Use device_iommu_capable()

Use the new interface to check the capability for our device
specifically.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Message-Id: <548e316fa282ce513fabb991a4c4d92258062eb5.1654688822.git.robin.murphy@arm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>