kvm tools: Introduce -net {user, tap, none} options for virtio net

kvm tools: Introduce -net {user, tap, none} options for virtio net

Use '-net user' to enable user model network, which enables plain user
without speical privileges to use network in guest.

Use '-net tap' to enable tap based network, which requres special
privileges.

Use '-net none' to disable virtio net.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>