vl: initialize displays _after_ exiting preconfiguration

Due to the renumbering of text consoles when graphical consoles are
created, init_displaystate must be called after all QemuConsoles are
created, i.e. after devices are created.

vl.c calls it from qemu_init_displays, while qmp_x_exit_preconfig is
where devices are created.  If qemu_init_displays is called before it,
the VGA graphical console does not come up.

Reported-by: Howard Spoelstra <hsp.cat7@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>