Add lib/base.sh functions to find physical disks from a block device

I have a partman-cros package that adds support for the special ChromeOS Kernel GPT partitions that Chromebook firmware/bootloader uses to boot. These are some helper functions that I use in that package to check if the partitions are on the same physical GPT disk that holds the root partition (which may be under LVM/encryption). I can keep them in partman-cros but I thought they may be general enough to go into partman-base, so filing this MR.