hw/xen: Fix broken check for invalid state in xs_be_open()

hw/xen: Fix broken check for invalid state in xs_be_open()

Coverity points out that if (!s && !s->impl) isn't really what we intended
to do here. CID 1508131.

Fixes: 032475127225 ("hw/xen: Add emulated implementation of XenStore operations")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230412185102.441523-6-dwmw2@infradead.org>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>