tests/qtest: enable tests for virtio-scmi

tests/qtest: enable tests for virtio-scmi

We don't have a virtio-scmi implementation in QEMU and only support a
vhost-user backend.  This is very similar to virtio-gpio and we add the same
set of tests, just passing some vhost-user messages over the control socket.

Signed-off-by: Milan Zamazal <mzamazal@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230628100524.342666-4-mzamazal@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>