libelf: Tidy up logging and remove dependency on stdio.

libelf: Tidy up logging and remove dependency on stdio.

libelf now permits callers to specify logging callback functions,
rather than a FILE*.  libelf's non-Xen callers are all libxc users, so
the stdio dependency and the default logging callback function (which
calls vfprintf) is now in libxc.

Xen's use of libxc is unaffected in this patch.

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