The USB network interface has a single buffer which the guest reads
from. This patch prevents multiple calls to usbnet_receive() from
clobbering the input buffer. Instead we queue packets until buffer
space becomes available again.
This is inspired by virtio-net and e1000 rxbuf handling.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
190563f9a9