net: intorduce qemu_del_nic()

To support multiqueue nic, this patch separate the nic destructor from
qemu_del_net_client() to a new helper qemu_del_nic() since the mapping bettween
NiCState and NetClientState were not 1:1 in multiqueue. The following patches
would refactor this function to support multiqueue nic.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>