Why just this one?

There are LOADS of asserts in libqb, some are OK and some may be
overkill. This one in particular is causing CI failures
and so annoys me more than the rest.

This seems to be needed for newer versions of libxml
but shouldn't break older ones (CI to confirm!)

* log: lower IPC connection issues to info level

... in handle_new_connection(). The caller has better context for whether a
problem merits a warning or error, and the function's return code is
sufficiently descriptive to do so. Some problems may be expected or able to be
worked around.

For example, Pacemaker's crm_mon attempts to contact pacemakerd IPC. On a
Pacemaker Remote node, that IPC will be unavailable, and crm_mon can check the
libqb return code to detect and handle that situation gracefully.

* log: lower some ringbuffer debug messages to trace level

They're rather noisy, with every shm-based IPC connection generating multiple
obscure messages like:

  debug: shm size:1048589; real_size:1052672; rb->word_size:263168

and every disconnect generating the rather unhelpful:

  debug: qb_ipcc_disconnect()

along with multiple messages like:

  debug: Closing ringbuffer: /dev/shm/qb-10986-11014-34-26VRvs/qb-request-cmap-header

All of these seem appropriate to trace level.