CA-138289: produce ring statistics in /dev/shm

blkback maintains shared ring statistics in a file in sysfs
(vbd-domid-devid/io_ring). This patch implements the same functionality
in tapdisk: tapdisk exports the same information under a new file in
/dev/shm (e.g /dev/shm/vbd3-domid-devid/io_ring), once every thirty
seconds for performance improvements. NB we don't deal with other
processes touching these files.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>