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