Re-arrange code

Move the definition of some inline functios higher up in the source
code so the declaration could be removed.

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