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"