Fix get_available_resource docstrings

libvirt's get_available_resource docstring had incorrect information,
this patch corrects it and clarifies the base, xenapi, and vmware docstrings as
well. Other drivers don't have long docstrings with the same issue.

Part of bp no-compute-fanout-to-scheduler

Change-Id: Iac943a7fa70337fbb0f742553a0e064511f0c265
Merge "Fix get_available_resource docstrings"