libxl: Rename libxl IDL infrastructure.

libxl: Rename libxl IDL infrastructure.

Originally libxltypes.py provided the infrastructure and libxl.idl provided the
specific types.

In 23887:a543e10211f7 libxl.idl became libxl_types.idl (to allow for
libxl_types_internal.idl) which means we now have libxl_types.FOO and
libxltypes.FOO providing different things and annoying people in tab
completion.

Rename the infrastructure as idl.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>