utils: add a function to validate a GUID string
generator: add a GUID parameter type

At the moment it is basically the change as String, and it is mapped as
if it was such.
generator: generate code for parameter validation

Implemented only in the C output, since every binding uses it anyway,
and just for the "GUID" type (since its format is well-known).
actions/part_set_gpt_type: set type of "guid" parameter as "GUID" (RHBZ#1008417).

Switch the type of the "guid" parameter from "String" to "GUID"; this
adds the validation of the GUID as such, rejecting straight away invalid
GUIDs which otherwise could be handled badly by low-level tools (such as
sgdisk).

Add a couple of easy tests (taken from RHBZ#1008417) to
part_set_gpt_type about this.