qemu-command: use -net socket, fd= with slirp-helper

qemu-command: use -net socket, fd= with slirp-helper

If a slirp-helper is associated with a network interface (after
probing & preparing succesfully), pass the socket fd to QEMU and use
"-net socket,fd=".

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>