support a configurable libvirt injection partition

This is useful if all guest images have the same structure,
and the root partition is not the first partition.

This is also handy to enable inspection in libguestfs,
which can handle disparate and complicated image layouts.

In future we may change to a StrOpt to support
searching by partition label.

Change-Id: Ie94d61bec8fe4b41d6d2d6d3efa9a4364cf027fe