vhost-user-rng: Back up vqs before cleaning up vhost_dev

vhost-user-rng: Back up vqs before cleaning up vhost_dev

vhost_dev_cleanup() clears vhost_dev so back up its vqs member to free
the memory pointed by the member.

Fixes: 821d28b88f ("vhost-user-rng: Add vhost-user-rng implementation")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20230130140516.78078-1-akihiko.odaki@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit f0dac71596d4b87a1a77d1f4efb6a6adb4730d7b)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>