vl: Fix exit status for -drive format=help

Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20181017082702.5581-35-armbru@redhat.com>

vl.c

@@ -4398,7 +4398,7 @@ int main(int argc, char **argv, char **envp)
     if (qemu_opts_foreach(qemu_find_opts("drive"), drive_init_func,
                           &machine_class->block_default_type, &error_fatal)) {
         /* We printed help */
-        exit(1);
+        exit(0);
     }
 
     default_drive(default_cdrom, snapshot, machine_class->block_default_type, 2,