cgpt: add edit command

This change adds a command to cgpt to
change the GUID of the drive.

BRANCH=none
BUG=None
TEST=Compiled and ran utility to verify that GUID changes.
Also verified that the new and existing tests completed
successfully.

Change-Id: Ia8a815447509626312e2b06c6f293901290c73c3
Signed-off-by: Matt Delco <delco@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1171834
Reviewed-by: Julius Werner <jwerner@chromium.org>