qemu-common.h: Make qemu_init_vcpu() stub static inline

Turn the *-user macro into a no-op inline function to avoid
unused-variable warnings and band-aiding #ifdef'ery.

This allows to drop an #ifdef for alpha and avoids more for unicore32
and other upcoming trivial realizefn implementations.

Suggested-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>