accel/xen: Fix xen_enabled() behavior on target-agnostic objects

accel/xen: Fix xen_enabled() behavior on target-agnostic objects

CONFIG_XEN is generated by configure and stored in "config-target.h",
which is (obviously) only include for target-specific objects.
This is a problem for target-agnostic objects as CONFIG_XEN is never
defined and xen_enabled() is always inlined as 'false'.

Fix by following the KVM schema, defining CONFIG_XEN_IS_POSSIBLE
when we don't know to force the call of the non-inlined function,
returning the xen_allowed boolean.

Fixes: da278d58a092 ("accel: Move Xen accelerator code under accel/xen/")
Reported-by: Paul Durrant <pdurrant@amazon.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20200804074930.13104-2-philmd@redhat.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>