xenapi: boot from volume without image_ref

Currently xenapi does not deal well with image_meta that does not
contain "disk_format". This happens when we get image_meta from an
associated volume, rather than from glance.

Change-Id: Id673158442fde27e8d468ca412c9bd557a886e6b
Closes-Bug: #1294069
Merge commit 'refs/changes/52/81252/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch