lib: printf code %pUs for GUID text representation

In different places text representations are used for GUIDs, e.g.

* command efidebug
* command part list for GPT partitions

To allow reducing code duplication introduce a new printf code %pUs.
It will call uuid_guid_get_str() to get a text representation. If none is
found it will fallback to %pUl and print a hexadecimal representation.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>