Virtio network device

Virtio network device

This patch implements the backend support for the virtio network device.  The
device is optimized for virtualized environments by limiting the number of
guest=>host transitions per-packet.  In the best case, the number of
transitions per-packet is < 1.

With some further optimizations, I have been able to obtain 1.5gbit/sec
host=>guest with this driver (compared to the 90mbit/sec from the rtl8139
card).  This requires additional patches not present in this series.

[dor: fix tx race with unload]

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Dor Laor <dor.laor@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>