libxl, tools/python: Add libxl python binding

libxl, tools/python: Add libxl python binding

Introduce python binding for libxl. The binding is not yet complete but
serveral methods are implemented and tested. Those which are implemented
provide examples of the two or three basic patterns that most future
methods should follow.

Over 5,000 lines of boilerplate is automatically generated to wrap and
export all relevant libxl structure definitions. There are a few places
where such code cannot be fully auto-generated and special hooks are
declared and stubbed where, for example, conversion between
libxl_file_reference and a python file object is required.

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