Support explicit user claim that a device is BIOS-visible.

	Support explicit user claim that a device is BIOS-visible.

	* grub-core/kern/emu/getroot.c (grub_util_get_dev_abstraction):
	Return GRUB_DEV_ABSTRACTION_NONE if device is in device.map.
	* grub-core/kern/emu/hostdisk.c
	(convert_system_partition_to_system_disk): Support mdX.
	(find_system_device): New parameter add. All users updated.
	(grub_util_biosdisk_is_present): New function.
	* include/grub/emu/hostdisk.h (grub_util_biosdisk_is_present): New
	proto.