vhost: support PACKED when setting-getting vring_base

Use the right structs for PACKED or split vqs when setting and
getting the vring base.

Fixes: 4c8cf318

 ("vhost: introduce vDPA-based backend")
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Message-Id: <20230424225031.18947-3-shannon.nelson@amd.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>