xen: expand BALLOON_MEMORY_HOTPLUG description

To mention it's also useful for PVH or HVM domains that require
mapping foreign memory or grants.

[boris: "non PV" instead of "translated" at Juergen's request]

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20200324150015.50496-1-roger.pau@citrix.com
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>