Add "dmctl uuid" command

This makes looking up device UUIDs a bit easier, as rather than depending on
the device mapper's sysfs layout we can depend on dmctl.  There's some
associated libdm plumbing, but the UUID was already pretty much availiable.

Test: I just ran this by hand.
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: I7028eec6ab04601308047b67057e51a0ff44c0a7