Fix implicit fallthrough warning on new gcc
    
    The option -Wimplicit-fallthrough was added to -Wall recently which
    generates a few warnings. Based on the comment above the switch, the
    fallthrough is on purpose so let's add a comment to avoid the following
    warnings.
    
    ovirt-foreign-menu.c: In function 'ovirt_foreign_menu_next_async_step':
    ovirt-foreign-menu.c:293:12: warning: this statement may fall through
             if (menu->priv->api == NULL) {
                ^
     ovirt-foreign-menu.c:297:5: note: here
          case STATE_VM:
               ^~~~
    ovirt-foreign-menu.c:298:12: warning: this statement may fall through
             if (menu->priv->vm == NULL) {
                ^
    ovirt-foreign-menu.c:302:5: note: here
         case STATE_STORAGE_DOMAIN:
              ^~~~
    ovirt-foreign-menu.c:303:12: warning: this statement may fall through
             if (menu->priv->files == NULL) {
                ^
    ovirt-foreign-menu.c:307:5: note: here
         case STATE_VM_CDROM:
              ^~~~
    
    ovirt-foreign-menu.c:308:12: warning: this statement may fall through
             if (menu->priv->cdrom == NULL) {
                ^
    ovirt-foreign-menu.c:312:5: note: here
       case STATE_CDROM_FILE:
            ^~~~
    
    Signed-off-by: Victor Toso <victortoso@redhat.com>
    Acked-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>