vhost-user: delete net client if necessary
    
    As qemu_new_net_client create new ncs but error happens later,
    ncs will be left in global net_clients list and we can't use them any
    more, so we need to cleanup them.
    
    Cc: qemu-stable@nongnu.org
    Signed-off-by: linzhecheng <linzhecheng@huawei.com>
    Signed-off-by: Jason Wang <jasowang@redhat.com>
    (cherry picked from commit c67daf4a24442d1bb404a11a6a54dc45ea10f234)
     Conflicts:
    	net/vhost-user.c
    * drop functional dep on 4d0cf552
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>