CA-146166: don't create io_ring uninitialised

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Flavien Quesnel <flavien.quesnel@citrix.com>

GitHub: closes #132 on xapi-project/blktap
CA-147141: Fix includes in tapdisk-interface.h

As tapdisk-interface.h was referencing to types which were not defined,
so automatic mock creation failed. With this fix, the file should
compile on its own.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Move declarations to source file

These inline methods were declared in the header file, and the unit
testing framework failed to create mock implementations. Moving the
declarations to the source file to avoid this problem.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Add example noop unit tests

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Test tapdisk_xenblkif_complete_request

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Rename test methods, add documentation

blktap is using the underscore convention, so using the same notation in
the tests as well. Also added some comments to explain the tests.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>