Move all tap-related functionality to the hv_kvm.netdev submodule. We
rename _OpenTap to OpenTap, since it will now be used as a public
function.

Also, change the hv_kvm tests to import the new code.

Signed-off-by: Apollon Oikonomopoulos <apoikos@gmail.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>