vhost: don't assume opaque is a fd, use backend cleanup

vhost: don't assume opaque is a fd, use backend cleanup

vhost-dev opaque isn't necessarily an fd, it can be a chardev when using
vhost-user. Goto fail, so vhost_backend_cleanup() is called to handle
backend cleanup appropriately.

vhost_set_backend_type() should never fail, use an assert().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>