virt: vbox: Use fallthrough;

Convert the various uses of fallthrough comments to fallthrough;

Done via script
Link: https://lore.kernel.org/lkml/b56602fcf79f849e733e7b521bb0e17895d390fa.1582230379.git.joe@perches.com/

And by hand:

drivers/virt/vboxguest/vboxguest_core.c has a fallthrough comment outside
of an #ifdef block that causes gcc to emit a warning if converted in-place.

So move the new fallthrough; inside the containing #ifdef/#endif too.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/68773b4cd82288b78ca6fcde8c43e249a025378a.1584040050.git.joe@perches.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>