virtio: set default queue size to maximum

The virtio PCI transport requires that the queue_size configuration
value returns the maximum supported queue size on reset; it uses 0 to
indicate an unavailable queue.

Queue::size is write-only via the existing MmioDevice (the driver must
always write a queue size during initialization), so there should be no
difference in behavior when using MMIO virtio devices.

Change-Id: I5b77e0d84f0bc7b854e33aaeb34ff011af226103
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1237363
Reviewed-by: Dylan Reid <dgreid@chromium.org>

devices/src/virtio/queue.rs

@@ -234,7 +234,7 @@ impl Queue {
     pub fn new(max_size: u16) -> Queue {
         Queue {
             max_size: max_size,
-            size: 0,
+            size: max_size,
             ready: false,
             desc_table: GuestAddress(0),
             avail_ring: GuestAddress(0),