libvirt: setting a correct driver name for iscsi volumes

Setting LibvirtISCSIVolumeDriver class to be identified
as a block device to determine the correct driver name.

Change-Id: Idfb89d051ccadab1d4c4b05b817b085e094ff992
Closes-Bug: #1273496
Merge "libvirt: setting a correct driver name for iscsi volumes"