Support degraded ZFS arrays in "grub-probe -t device" resolution.
* grub-core/kern/emu/getroot.c (find_root_device_from_libzfs): When
the pool is an array of devices, iterate through it and return the
first device that passes a stat() test (instead of blindly returning
the first one).
* 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.