vl.c: Fix OpenBSD compilation issue due to namespace collisions

Machine rewriting added MACHINE() macro which is
already in use by other OpenBSD library.
Since qemu/sockets.h exposes the OpenBSD namespace,
the minimalistic approach is to add it as the first QEMU include.

Reported-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
