feat(partition): add a function to identify a partition by GUID

With the GPT partition scheme, a partition can be identified using
it's UniquePartitionGUID, instead of it's name. Add a function to
identify the partition based on this GUID value. This functionality is
useful in identification of a partition whose UniquePartitionGUID
value is known.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I543f794e1f7773f969968a6bce85ecca6f6a1659