[libvirt] [PATCH 5/5]: Export <device> when doing logical pool discovery

When doing logical pool discovery, it currently returns XML like: <sources><source><name>myvg</name></source></sources> However, for oVirt, we need to know the device name that corresponds to this logical volume group as well. Therefore, extend the virStorageBackendLogicalFindPoolSources function to return this information as well. This required pretty much a complete rewrite of the discovery function, since I needed to be able to add <device> sections to already existing XML nodes. For that reason, this code uses direct libxml2 tree API's to handle the parsing and the adding of nodes where necessary. Signed-off-by: Chris Lalancette <clalance@redhat.com>