fix mingw build failure

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170516052439.16214-1-kraxel@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>