Increase vfio-ioctls coverage

[sameo]

By adding unit tests. There currently are zero unit testing.

We won't be able to push that code upstream without at least some unit testing.

[sameo]

cc @zhabinecho

[zhabinecho]

Yeah, I'll cover it later.

[arronwy]

We found the unit tests for vfio-ioctls crate may require some real PCI devices. Current integration test of cloud-hypervisor vfio part use nested virtualization. For vfio-ioctls unit test we may need use a device which need exists in most of the platforms like USB? or through emulation? any suggestions?