Merge branch 'CA-134745' into xs64bit

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

GitHub: closes #85 on xapi-project/blktap
CA-135969: disable verbose messages in tapback

They seem to significantly delay tapback's execution.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Mate Lakat <mate.lakat@citrix.com>
CA-135969: don't log to separate file, use syslog instead

Not logging to syslog leads to nasty lock ups and long delays.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Mate Lakat <mate.lakat@citrix.com>
Merge branch 'CA-135969' into xs64bit

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

GitHub: closes #90 on xapi-project/blktap
Revert "Merge branch 'CA-135969' into xs64bit"

This reverts commit b1ea896f36e6c2c0bca8496fcb668a95411c09cb, reversing
changes made to 246b35670f68410c1f589c99bbddc69858f59032. Revert required
to fix BVT.
CA-137990: 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>