libvirt: sync get_available_resources and get_host_stats

get_available_resources is actually used by the scheduler while
get_host_stats was previously used, and is still used for host
capabilities.  This patch makes get_available_resources and
get_host_stats use the same logic to clean up the code. As part of
making them use the same logic, some of the unused data returned from
get_host_stats is changed to be what get_available_resources expects.
This is also in preparation for removing the periodic RPC fanout from
compute nodes to the scheduler.

This patch cleans up libvirt and fake drivers only.  This is not needed
for other virt backends as this is a cleanup only.  Further cleanup of
libvirt and other drivers will happen after get_host_stats isn't used
for the compute fanout to schedulers.

Because this makes a change to the fake driver, several api samples
needed to be changed as well. The fake driver is changed so the
test_virt_driver tests can continue to be used

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

Change-Id: I1eec5c117a1cb0490e9f9c09e731909bc31698a9
Merge "libvirt: sync get_available_resources and get_host_stats"