xtl: New xentoollog mini-library.

xtl: New xentoollog mini-library.

We provide a new header file "xentoollog.h" which defines an interface
that libraries and applications can use for logging.  This avoids
having to wrap each library's log callbacks up, massage arguments to
log callbacks, and so on.

The library's .o files are within libxc to avoid having to create a
separate lib*.a, but callers do not need to #include xenctrl.h and it
should be regarded as a separate API.

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