xen/events/fifo: Consume unprocessed events when a CPU dies

When a CPU is offlined, there may be unprocessed events on a port for
that CPU.  If the port is subsequently reused on a different CPU, it
could be in an unexpected state with the link bit set, resulting in
interrupts being missed. Fix this by consuming any unprocessed events
for a particular CPU when that CPU dies.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Cc: <stable@vger.kernel.org> # 3.14+
Signed-off-by: David Vrabel <david.vrabel@citrix.com>