xen: Rework WARN_ON() to return whether a warning was triggered

xen: Rework WARN_ON() to return whether a warning was triggered

So far, our implementation of WARN_ON() cannot be used in the following
situation:

if ( WARN_ON() )
    ...

This is because WARN_ON() doesn't return whether a warning has been
triggered. Such construciton can be handy if you want to print more
information and also dump the stack trace.

Therefore, rework the WARN_ON() implementation to return whether a
warning was triggered. The idea was borrowed from Linux

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>