tests: Fix race in misc:DdWatchCopy.testStop

Use a named pipe instead of an actual source file so we know no data
will ever be read.

Change-Id: I50e044ec2d949a95e52adcfbc0098a032fb7193d
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11564
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>