xen/xenbus: Allow watches discard events before queueing

If handling logics of watch events are slower than the events enqueue
logic and the events can be created from the guests, the guests could
trigger memory pressure by intensively inducing the events, because it
will create a huge number of pending events that exhausting the memory.

Fortunately, some watch events could be ignored, depending on its
handler callback.  For example, if the callback has interest in only one
single path, the watch wouldn't want multiple pending events.  Or, some
watches could ignore events to same path.

To let such watches to volutarily help avoiding the memory pressure
situation, this commit introduces new watch callback, 'will_handle'.  If
it is not NULL, it will be called for each new event just before
enqueuing it.  Then, if the callback returns false, the event will be
discarded.  No watch is using the callback for now, though.

This is part of XSA-349

Cc: stable@vger.kernel.org
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reported-by: Michael Kurth <mku@amazon.de>
Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>