sysemu/xen-mapcache: Check Xen availability with CONFIG_XEN_IS_POSSIBLE

"sysemu/xen.h" defines CONFIG_XEN_IS_POSSIBLE as a target-agnostic
version of CONFIG_XEN accelerator.
Use it in order to use "sysemu/xen-mapcache.h" in target-agnostic files.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20231114143816.71079-4-philmd@linaro.org>
1 file changed