virt: vbox: Move declarations of vboxguest private functions to private header

Move the declarations of functions from vboxguest_utils.c which are only
meant for vboxguest internal use from include/linux/vbox_utils.h to
drivers/virt/vboxguest/vboxguest_core.h.

Cc: stable@vger.kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>