libxenlight: Clean up logging arrangements

libxenlight: Clean up logging arrangements

* Introduce new variants of the logging functions which include
  errno values (converted using strerror) in the messages passed to
  the
  application's logging callback.

* Use the new errno-including logging functions everywhere where
  appropriate.  In general, xc_... functions return errno values or 0;
  xs_... functions return 0 or -1 (or some such) setting errno.

* When libxl_xs_get_dompath fails, do not treat it as an allocation
  error.  It isn't: it usually means xenstored failed.

* Remove many spurious \n's from log messages.  (The applications log
  callback is expected to add a \n if it wants to do that, so libxl's
  logging functions should be passed strings without \n.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>