xl: free bitmaps on exit

xl: free bitmaps on exit

Every invocation of xl via valgrind will show three leaks.
Since libxl_bitmap_alloc uses NOGC, the caller has to free the memory
after use. And since xl_ctx_free might be called before
parse_global_config, also move the libxl_bitmap_init calls into
xl_ctx_alloc.

Also move the call to atexit() after xl_ctx_alloc, because the latter is
also called again in postfork.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wei.liu2@citrix.com>