VMware: consolidate datastore code

This is largely a straight-forward code movement.
Given that we have utility classes/code in ds_util.py, it makes sense
to move the remaining datastore-related code in vm_util.py to ds_util.py
as well.

partial blueprint vmware-spawn-refactor
Change-Id: I2ede44306acf91a13f2ff6f0d8f210642a1c1e69
libvirt: speed up _get_disk_over_committed_size_total method

Switch the _get_disk_over_committed_size_total method to
use the new list_instance_domains method.

Blueprint: libvirt-domain-listing-speedup
Change-Id: I4ef00ea5584eda14c4781d3fbf8bb737f3abdc90
Merge "libvirt: speed up _get_disk_over_committed_size_total method"
Merge "VMware: consolidate datastore code"