XenAPINFS: Create volume from image (generic)

Fixes bug 1130706

Related to blueprint xenapinfs-glance-integration

This patch enables users to use any images recognised by qemu-img to
create volumes on XenAPINFS.

Change-Id: I9dd8ec237309da82ec7f73db1acb48e5b7411c9e
Merge "XenAPINFS: Create volume from image (generic)"