powervm: add test case for get_available_resource

Change If9eee203b3cbb12963dc55e4215d206cb5d81264 is modifying the
get_available_resource method but didn't have a test to cover it,
because it never existed.  This patch adds the test.

Change-Id: Ifbef914f1b110b16fa98fdfec8cdcf5ad0a75a8e
Merge "powervm: add test case for get_available_resource"