[libvirt] [PATCH]: Add default pool types

As suggested by danpb, to fix up the regression caused by last week's VIR_ENUM cleanup patch, add a ".defaultFormat" member to .poolOptions. In storage_conf.c, if virXPathString(/pool/source/format/@type) returns NULL, then set the pool type to .defaultFormat; otherwise, lookup the type via formatFromString. Signed-off-by: Chris Lalancette <clalance(a)redhat.com&gt;