kvm tools: Guest kernel compatability

The problem of users running kernel which wasn't built for kvm tools is
somewhat common. Usualy the result is a panic during boot and a user whos'
stuck and doesn't know what to do next.

Compatability checks try to solve this issue by giving the user a meaningfull
error message which can explain whats going on and how he can fix it.

The idea is to register the device during device init and unregister the
device when the device has been 'used'. In case the kernel didn't access
the device before the guest is shut down, the host will notify the user that
something is wrong.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>