Since there is code using functions from the libxml library,
libvirt_conf should have that in LIBADD so it can be linked against
even without libvirt_util (which usually deals with the error itself,
since libvirt_util has libxml in LIBADD).  The same applies to
storage_backend.c.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>