When the network interface is of "user" type, and QEMU has the "-net
socket,fd=" datagram support, call qemuInterfacePrepareSlirp() to
probe and associate a slirp-helper with the interface.

The usage of automated slirp-helper can be prevented with
disableSlirp (in particular when resuming a
VM that didn't start with slirp-helper before).
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>