Fix for qemu-nbd hang

NBD device once used seem to run into intermittent trouble when
used with mount repeatedly. Adding code to explicitly flush the
device buffers using 'blockdev --flushbufs'.

Closes-Bug: #973413
Partial-Bug: #1254890
Change-Id: I2b7053b9a069d6e82f6f6baf9ad480efa4388d91