Description of problem:
1. For sgdisk, -t, --typecode=partnum:{hexcode|GUID}, it can accept hexcode as input
   For guestfish, we'd better update the helpout of part-set-gpt-type according to sgdisk, tells that hexcode is valid, command does not return error.

2. If gives a non-guid format value, sgdisk will give a default guid, like:
# sgdisk /dev/sdb -t 1:f
Exact type match not found for type code 000F; assigning type code for
'Linux filesystem'

For part-set-gpt-type, if gives a non-guid format value for gpt, the default value is returned, it's equal to mbr's guid (0FC63DAF-8483-4772-8E79-3D69D8477DE4).
it does not match gpt, we also need update helpout for this

 
Version-Release number of selected component (if applicable):
libguestfs-1.22.6-8.el7.x86_64


How reproducible:
100%

 
Steps to Reproduce:
1. check for mbr
><fs> part-disk /dev/sda mbr
><fs> part-get-gpt-type /dev/sda 1
0FC63DAF-8483-4772-8E79-3D69D8477DE4

2. check for gpt
><fs> part-disk /dev/sda gpt
><fs> part-get-gpt-type /dev/sda 1
EBD0A0A2-B9E5-4433-87C0-68B6B72699C7

3. set an invalid guid for gpt
><fs> part-set-gpt-type /dev/sda 1 f
><fs> part-get-gpt-type /dev/sda 1
0FC63DAF-8483-4772-8E79-3D69D8477DE4


Actual result:


Expect result:


Additional info: