qmp: Clarify memory backend properties returned via query-memdev

qmp: Clarify memory backend properties returned via query-memdev

We return information on the currently configured memory backends and
don't configure them, so decribe what the currently set properties
express.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Suggested-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com> for memory backend and machine core
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210510114328.21835-12-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>