[libvirt] [PATCH 3/5]: Set format type to LVM2 on NULL format

Again because of my generic VIR_ENUM_IMPL patch last week, I unwittingly caused a regression in the storage_backend_logical driver. Previously, if you submitted logical pool XML that had no <source><format type='lvm2'/> string, it would just default to VIR_STORAGE_POOL_LOGICAL_LVM2. This would succeed just fine and go on with life. This is no longer the case, and now XML without the format tag fails to define. To maintain backwards compatibility with already existing XML that expects this, add a compatibility wrapper to return VIR_STORAGE_POOL_LOGICAL_LVM2 on an empty format tag. Signed-off-by: Chris Lalancette <clalance(a)redhat.com&gt;