xen: enable XEN_UNPOPULATED_ALLOC as part of xen.config

PVH dom0 is useless without XEN_UNPOPULATED_ALLOC, as otherwise it will
very likely balloon out all dom0 memory to map foreign and grant pages.

Enable it by default as part of xen.config.  This also requires enabling
MEMORY_HOTREMOVE and ZONE_DEVICE.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250514092037.28970-1-roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>