virtio: add vhost-user-base and a generic vhost-user-device

virtio: add vhost-user-base and a generic vhost-user-device

In theory we shouldn't need to repeat so much boilerplate to support
vhost-user backends. This provides a generic vhost-user-base QOM
object and a derived vhost-user-device for which the user needs to
provide the few bits of information that aren't currently provided by
the vhost-user protocol. This should provide a baseline implementation
from which the other vhost-user stub can specialise.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230710153522.3469097-8-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>