tools/libxc: fix error handling in xc_mem_paging_load

tools/libxc: fix error handling in xc_mem_paging_load

xc_mem_paging_load() does not pass errors in errno and the actual
errno from xc_mem_event_control() is overwritten by munlock().
xenpaging_populate_page() needs to check errno, but with the switch to
xc_mem_paging_load() it could not receive ENOMEM anymore.

Update xc_mem_paging_load() to return -1 and preserve errno during
munlock().

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Committed-by: Keir Fraser <keir@xen.org>