Add VIRTIO_NET_F_SPEED_DUPLEX (4) feature bit

Hi,

We have found it useful to have the virtio net device export a linkspeed
and duplex setting, such that a guest linkspeed and duplex settings can
be set automatically from the host side. We have some guest software
that relies on this setting, and fails without a valid value provided.
Thus, I would like to propose adding a new feature bit:

VIRTIO_NET_F_SPEED_DUPLEX (4) Device provides a linkspeed and duplex setting

I have provided sample qemu hypervisor and linux kernel guest code to
implement this feature here:

http://lkml.iu.edu/hypermail/linux/kernel/1712.1/05692.html

Thanks,

-Jason