qmp: Include "reserve" property of memory backends

Let's include the new property. Instead of relying on CONFIG_LINUX,
let's try to unconditionally grab the property and treat errors as
"does not exist".

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-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: <20210510114328.21835-15-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>