CA-140343: Fix error sign

Incorrect error sign shows "Unknown error 18446744073709551614"
Fixed error sign to get more meaningful error string.

Signed-off-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
Reviewed-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>

GitHub: closes #117 on xapi-project/blktap
CP-9457: write tapdisk PID in kthread-pid

This imitates blkback, allowing other components to use the process ID
of the application responsible for the data path as they please, e.g.
the toolstack using ionice(1) for QoS.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>

GitHub: closes #121 on xapi-project/blktap
CA-144882: don't complain about ENOENT when cleaning up the stats file

And ignore, but do log, errors when cleaning up the stats file.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #123 on xapi-project/blktap
CA-136792: produce raw 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. Unlike
blkback, the metric are in raw binary format, as this is the most
efficient way for producing them. The format of the statistics is
defined in /usr/include/blktap3.h.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>

GitHub: closes #97 on xapi-project/blktap
CA-136792: reword function and clean up FIXME

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