ensure zeros are written out when clearing volumes

Note O_DIRECT is _not_ used at the end of devices to
avoid issues with odd sized blocks etc. so instead
we arrange to have dd issue an fdatasync()
to ensure the data is persisted, lest it be discarded
from the write cache when the device is unprovisioned.

* nova/virt/libvirt/utils.py (clear_logical_volume): Add 'conv=fdatasync'
to the dd option list if O_DIRECT isn't used when clearing.

Fixes bug: 1100363
Change-Id: I76789557754ebaeb6d52bb34548a2ef17808fbf6
Merge "ensure zeros are written out when clearing volumes"