Fix: libcrmcommon: Drop xmlRegisterNodeDefault()

...and xmlDeregisterNodeDefault().

Within libxml2, these are tagged with the "deprecated" compiler
attribute, causing builds on some bleeding-edge distributions (currently
at least debian-experimental) to fail.

Instead, allocate and free private data as needed when creating and
freeing XML nodes.

Fixes T838

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>