BUG-994: QName.toString() performance

This fixes an obvious mistake: QName.toString() was not using the
cached, pre-formatted revision string.

Change-Id: I9cae1e97e038eaad01c7ce5d3b47c6c1b94cd575
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 124e7a75cd8e602271f98c36d35e5d2958fd8a09)
Signed-off-by: Robert Varga <rovarga@cisco.com>