kvm tools: virtio-net init/exit

kvm tools: virtio-net init/exit

Make the init/exit of virtio-net self-contained, so the global init code
won't need to check if it was selected or not.

This also moves the bulk of the net-specific initialization code, including
the parser, into virtio-net itself.

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