Fix use of libvirt_disk_prefix

The code supporting libvirt_disk_prefix was dropped during the
refactoring work in commit 7be531fe.  This patch re-enables
support for this option, along with a test case to verify it.

Change-Id: I84f059e2b35d9237b799236032de5c272a03a451
Merge "Fix use of libvirt_disk_prefix"