Fix the is_volume_backed_instance check

Changing the check for volume backed instance to handle instances with
no root_device_name

Fixes LP bug #1197237

Change-Id: If0e2dff98a55ed1addf8a010849389f85db427f6
Merge "Fix the is_volume_backed_instance check"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp