libfdisk: propagate assign-device operation to parent

Let's make it possible to use fdisk_assign_device() for nested
contexts.

Signed-off-by: Karel Zak <kzak@redhat.com>
sfdisk: add --label-nested for hybrid GPT

regular disk label:
	# sfdisk --list /dev/sdb
	..
	Disklabel type: gpt
	Disk identifier: 9DF9A9F1-0654-4E7A-9A5E-36E66D60FD79

	Device     Start    End Sectors Size Type
	/dev/sdb1   2048  22527   20480  10M Linux filesystem
	/dev/sdb2  22528  43007   20480  10M Linux swap
	/dev/sdb3  43008 204766  161759  79M Linux filesystem

nested (PMBR):
	# sfdisk --list --label-nested dos /dev/sdb
	...
	Disklabel type: dos
	Disk identifier: 0x00000000

	Device     Boot Start    End Sectors  Size Id Type
	/dev/sdb1           1 204799  204799  100M ee GPT

and for example:
        # sfdisk --label-nested dos /dev/sdb <<EOF
        1, 2047, ee
        , 10M, L
        , 10M, S
        ,,

creates hybrid GPT (PMBR partitions point to the same location as GPT)

Signed-off-by: Karel Zak <kzak@redhat.com>