xen: add macro for version number string

xen: add macro for version number string

On Arm64 Linux kernel prints log for Xen version number:

  Xen XEN_VERSION.XEN_SUBVERSION support found

The header file "xen/compile.h" is missed so that XEN_VERSION and
XEN_SUBVERSION are not defined, __stringify() wrongly converts them as
strings and concatenate to string "XEN_VERSION.XEN_SUBVERSION".

This patch introduces a string macro XEN_VERSION_STRING, we can directly
use it as version number string, as a result it drops to use of
__stringify() to make the code more readable.

The change has been tested on Ampere AVA Arm64 platform.

Fixes: 5d797ee199b3 ("xen/arm: split domain_build.c")
Suggested-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>