tests: libxl: Mock xs_open and xs_close

tests: libxl: Mock xs_open and xs_close

The Xen-related unit tests are failing against the recently released
Xen 4.15. Xen commit 90c9f9f4dd changed the implementation of
libxl_ctx_alloc to use xs_open instead of xs_daemon_open. libvirt has
already mocked xs_daemon-{open,close} and others to allow using libxl
in confined build environments. This patch adds xs_{open,close} to the
list of functions mocked in libxlmock.c

https://github.com/xen-project/xen/commit/90c9f9f4ddd55e11be0506bff10c6237507c6e0d

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>