test guest disks operations with existing userid
test guest disks operations with existing userid

Use the userid that created in setupClass to test disk related
testcases.

Change-Id: I87f3649a6228aded5dcf98cb8248ad188c5afa7d

mfcloud/python-zvm-sdk,mfcloud/python-zvm-sdk,mfcloud/python-zvm-sdk,mfcloud/python-zvm-sdk