Created by copying from qemuxml2argv-disk-drive-v2-wb.*, then
s/writeback/directsync/.  Hopefully this matches Osier's intentions.

* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-cache-directsync.args:
* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-cache-directsync.xml:
Add missing files needed by 'make check'.