[NET] back: Disable packet queuing when a client has no receive buffers.

[NET] back: Disable packet queuing when a client has no receive buffers.

This turns out to be dangerous as packets can be queued indefinitely,
introducing resource dependencies between guests (if the queued
packets originate from another virtual machine). For example, this
can prevent interfaces from being torn down and hence defunct
virtual machines from being destroyed.

The queuing functionality is retained as a module option, but a
warning is printed if it is enabled. It is not intended for general use!

Signed-off-by: Keir Fraser <keir@xensource.com>