net/virtio-user: do not stop stopped device again

net/virtio-user: do not stop stopped device again

Without this change, virtio-user still works, but it will show
annoying error messages like this on shutdown:

vhost_kernel_set_backend(): VHOST_NET_SET_BACKEND fails, Operation not permitted
vhost_kernel_ioctl(): VHOST_RESET_OWNER failed: Operation not permitted

Fixes: e3b434818bbb ("net/virtio-user: support kernel vhost")
Fixes: 12ecb2f63b12 ("net/virtio-user: support memory hotplug")
Cc: stable@dpdk.org
Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>