x86_64: support sysupgrade config restore on PARTUUID mounted disks

Fix the move_config() procedure to properly find the corresponding disk if
the rootfs was mounted by PARTUUID instead of a device node.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42946 3c298f89-4303-0410-b956-a3cf2f4a3e73