net/virtio: fix Tx completed mbuf leak on device stop

Free Tx completed mbufs on device stop. Not completed Tx mbufs cannot be
freed since they are still in use.

Fixes: c1f86306a026 ("virtio: add new driver")
Cc: stable@dpdk.org
Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>