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