xen: check required Xen features

Linux kernel is not supported to run on Xen versions older than 4.0.

Add tests for required Xen features always being present in Xen 4.0
and newer.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210730071804.4302-2-jgross@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>