virtio: Decrement avail idx on buffer detach

virtio: Decrement avail idx on buffer detach

When detaching a buffer from a vq, the avail.idx value should be
decremented as well.

This was noticed by hot-unplugging a virtio console port and then
plugging in a new one on the same number (re-using the vqs which were
just 'disowned').  qemu reported

   'Guest moved used index from 0 to 256'

when any IO was attempted on the new port.

CC: [email protected]
Reported-by: juzhang <[email protected]>
Signed-off-by: Amit Shah <[email protected]>
Signed-off-by: Rusty Russell <[email protected]>