For the vhost-user net-card, it is allow to delete its
network backend while the virtio-net device still exists.
However, when the status of the device changes in guest,
QEMU will check whether the network backend exists, otherwise
it will crash.
So do not allowed to delete the network backend directly
without delete virtio-net device.
Signed-off-by: Jinhua Cao <caojinhua1@huawei.com>
