chardev: Use real-time clock for open timer

The vm clock may be stopped, and then we won't get open events anymore.
Seen with QMP sessions.
Reported-by: NDietmar Maurer <dietmar@proxmox.com>
Tested-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

qemu-char.c

@@ -134,9 +134,9 @@ static void qemu_chr_fire_open_event(void *opaque)
 void qemu_chr_generic_open(CharDriverState *s)
 {
     if (s->open_timer == NULL) {
-        s->open_timer = qemu_new_timer_ms(vm_clock,
+        s->open_timer = qemu_new_timer_ms(rt_clock,
                                           qemu_chr_fire_open_event, s);
-        qemu_mod_timer(s->open_timer, qemu_get_clock_ms(vm_clock) - 1);
+        qemu_mod_timer(s->open_timer, qemu_get_clock_ms(rt_clock) - 1);
     }
 }