Fix hang in virtio_driver::wait_for_queue

virtio_driver::wait_for_queue() would often hang in a memcached and
mc_benchmark workload, waiting forever for received packets although
these *do* arrive.

As part of the virtio protocol, we need to set the host notification
flag (we call this, somewhat confusingly, queue->enable_interrupts())
and then check if there's anything in the queue, and if not, wait
for the interrupt.

This order is important: If we check the queue and only then set the
notification flag, and data came in between those, the check will be
empty and an interrupt never sent - and we can wait indefinitely for
data that has already arrived.

We did this in the right order, but the host code, running on a
different CPU, might see memory accesses in a different order!
We need a memory fence to ensure that the same order is also seen
on other processors.

This patch adds a memory fence to the end of the enable_interrupts()
function itself, so we can continue to use it as before in
wait_for_queue(). Note that we do *not* add a memory fence to
disable_interrupts() - because no current use (and no expected use)
cares about the ordering of disable_interrupts() vs other memory
accesses.

drivers/virtio-vring.cc

@@ -90,6 +90,7 @@ namespace virtio {
         trace_virtio_enable_interrupts(this);
         _avail->enable_interrupt();
         set_used_event(_used_ring_host_head, std::memory_order_relaxed);
+        std::atomic_thread_fence(std::memory_order_seq_cst);
     }
 
     bool