Add unix_listen_opts().  Does the same as unix_listen(), but uses
QemuOpts.  unix_listen() is a compatibility wrapper for
unix_listen_opts() now and should go away some day.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>