Hyper-V: Skip logging out in-use targets

As most of the Cinder volume drivers that use iSCSI export one LUN
per iSCSI target, the Hyper-V driver was logging out the according
target each time a volume was disconnected.

This causes issues with some volume drivers that use multiple LUNs
per iSCSI target and may disconnect in-use targets.

This patch fixes the issue by checking if there are other LUNs
being used, disconnecting the iSCSI targets otherwise.

Change-Id: Ida08e7129a7b7940ddea488488cccc7571a43b76
Closes-Bug: #1372815

Conflicts:
	nova/tests/virt/hyperv/test_hypervapi.py
	nova/virt/hyperv/volumeops.py
Hyper-V: cleanup basevolumeutils

This patch implements a TODO that was there for some time now,
using Regex in order to retrieve the disk number from a physical
drive path.

Also, some WQL queries have been replaced with filters, which
are cleaner and safer.

Change-Id: I6ee54b72b142cffc271b271937f1b3f9b6b6dbdf

(cherry picked from 1e4cee2a67b5adf1239db55d68c8f212c917f934)

Conflicts:
	nova/tests/virt/hyperv/test_basevolumeutils.py