MAINTAINERS: Update Xen maintainership

Due to time constraints I am stepping down as maintainter. I will stay
as reviewer for x86 code (for which create a separate category).

Stefano is now maintainer for Xen hypervisor interface and Oleksandr has
graciously agreed to become a reviewer.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Link: https://lore.kernel.org/r/1653674225-10447-1-git-send-email-boris.ostrovsky@oracle.com


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