virtio: create admin queues alongside other virtqueues

Admin virtqueue is just another virtqueue nothing that special about it.
The current implementation treats it somehow separate though in terms
of creation and deletion. Unify the admin virtqueue creation and
deletion flows to be aligned with the rest of virtqueues, creating
it from vp_find_vqs_*() helpers. Let the admin virtqueue to be deleted
by vp_del_vqs() as the rest.

Call vp_find_one_vq_msix() with slow_path argument being "true" to make
sure that in case of limited interrupt vectors the config vector is used
for admin queue.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20240716113552.80599-10-jiri@resnulli.us>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>