Build the whole blktap

gntdev.h was a custom one, so for now including a copy here. Executing
make from the project's root, so the whole blktap is built.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
Only running integration tests if they exists

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