[settings] Add parsing for UUID and GUID settings types

[settings] Add parsing for UUID and GUID settings types

The ":uuid" and ":guid" settings types are currently format-only: it
is possible to format a setting as a UUID (via e.g. "show foo:uuid")
but it is not currently possible to parse a string into a UUID setting
(via e.g. "set foo:uuid 406343fe-998b-44be-8a28-44ca38cb202b").

Use uuid_aton() to implement parsing of these settings types, and add
appropriate test cases for both.

Signed-off-by: Michael Brown <mcb30@ipxe.org>