xen: specify support for EXPERT and DEBUG Kconfig options

xen: specify support for EXPERT and DEBUG Kconfig options

Add a clear statement about them, reflecting the current security
support status of Kconfig options (no changes to current policies).

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
CC: George.Dunlap@eu.citrix.com
CC: Ian.Jackson@eu.citrix.com
CC: jbeulich@suse.com
CC: andrew.cooper3@citrix.com
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Tim Deegan <tim@xen.org>
CC: Wei Liu <wei.liu2@citrix.com>
---
Changes in v7:
- talk about EXPERT and DEBUG rather than CONFIG_EXPERT and CONFIG_DEBUG