CA-136792: produce blkback-style stats

Produce I/O statistics in /dev/shm/vbd3-domid-devid. We use one file for
all seven counters as otherwise we're wasting too much memory. Stats are
updated once every thirty seconds to minimise impact on performance.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
I dont think this line is necessary.