vl.c: Guard against GThread double-initialization

GThread-ERROR **: GThread system may only be initialized once.
aborting...

Making the g_thread_init() call in vl.c conditional resolves an abort on
Mac OS X, where coroutine-gthread.c seems to call it before vl.c.

Reported-by: Juan Pineda <juan@logician.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Cc: Daniel P. Berrange <berrange@redhat.com>