Hyper-V: attach volumes via SMB

Currently, the Hyper-V driver supports attaching volumes only via
iSCSI. This patch adds support for attaching volumes using VHD or
VHDX images hosted on SMB shares.

Note that the according Cinder drivers which allow exporting
images through SMB shares have been already merged.

The workflow is simple: using the connection info, the driver
chooses the right volume driver, mounts the according SMB share
using the according credentials. The only thing that needs to be
done next is actually attach the volume to an instance using its
UNC path.

Change-Id: Id5de0e6843c1035235598a4d72a00b7555187fb4
Implements: blueprint hyper-v-smbfs-volume-support