CP-7800: revert fixes for tap-ctl race conditions

This revert the commits that fix the tap-ctl race conditions use retries
and timeouts, as that issue is properly fixed by CP-7800. The following
commits are reverted:
 * 0e3b72eb: "CAR-96/CA-128039: don't use uninitialised vars when checking for
             timeout"
 * 0e3dee2d: "CAR-96/CA-128039: don't check error field if for disk_info"
 * 7a99303a: "CAR-96/CA-128039: retry tap-ctl-info on EBUSY"
 * 9a9ca3b4: "CAR-96/CA-128039: correctly set error code when timing out"
 * a0046e5f: "CAR-96/CA-128039: retry connect to/disconnect from on EBUSY"
 * da6784c5: "CAR-96/CA-128039: fix tap_ctl_connect/disconnect_xenblkif error
              handling"
 * e1816cea: "Revert "CAR-96/CA-7800: mask conflicting commands instead of
             retrying""
 * ad30ba6b: "CAR-96/CA-7800: mask conflicting commands instead of retrying"

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
CP-7800: Include PID in tap-ctl log messages

This is not directly related to fixing the issue, but while debugging
the bug (using multiple, concurrent tap-ctl commands) we ran into the problem
of not being able to tell which tap-ctl is which.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
CP-7800: mask conflicting commands instead of retrying

The Storage Manager and tapback might concurrently issue exclusive
commands to tapdisk. tapdisk is not designed to handle concurrent
commands and if this occurs it fails the other command with EBUSY. We
address this issue by masking the event associated with the command that
would be rejecting while tapdisk is busy servicing the first command, and
unmasking it when the first command has finished.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
Merge CP-7800

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

GitHub: closes #73 on xapi-project/blktap