net: fix usbnet_receive() packet drops

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>
