Use gdbpy_enter in py-xmethod.c

Use gdbpy_enter in py-xmethod.c

This changes the remaining functions in py-xmethod.c to use
gdbpy_enter; using gdbpy_ref and unique_xmalloc_ptr as
appropriate.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/py-xmethods.c (gdbpy_get_xmethod_result_type): Use
	gdbpy_enter, gdbpy_ref, unique_xmalloc_ptr.
	(gdbpy_invoke_xmethod): Use gdbpy_ref, gdbpy_enter.
	(gdbpy_get_xmethod_arg_types): Use gdbpy_ref,
	unique_xmalloc_ptr.
	(gdbpy_get_xmethod_arg_types): Use gdbpy_ref, gdbpy_enter.