Add support for fd=name to tap and socket networking

This allows a program to initialize a host networking device using a
file descriptor passed over a unix monitor socket.

The program must first pass the file descriptor using SCM_RIGHTS
ancillary data with the getfd monitor command. It then may do
"host_net_add tap fd=name" to use the named file descriptor.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
