Calculate stopped instance's disk sizes for disk_available_least

Calculate stopped instance's disk sizes for disk_available_least

disk_available_least is a free disk size information of hypervisors.
This is calculated by the following fomula:

disk_available_least = <free disk size> - <Total gap between virtual
disk size and actual disk size for all instances>

But stopped instance's virtual disk sizes were not calculated
after merging following patch in Juno cycle:
https://review.openstack.org/#/c/105127

So disk_available_least might be larger than actual free disk size.
As a result, instances might be scheduled beyond the actual free
disk size if stopped instances were on a host.

This patch fix it.
Stopped instance's disks will be calculated after merging this patch.

Change-Id: I8abf6edfa80e3920539e4f6d4906c573f9543b91
Closes-Bug: #1693679
(cherry-picked from commit 3342215034c7a7a938c497c39f6405763201a261)