GET_VRING_BASE stops the queue, clearing the call and kick fds. However,
we don't clear vring.avail. That means that if vu_queue_notify() is called
it won't realise the queue isn't ready and will die with an EBADFD.
We get this during migration, because for some reason, qemu reconfigures
the vhost-user device when a migration is triggered. There's a window
between the GET_VRING_BASE and re-establishing the call fd where the
notify function can be called, causing a crash.
Signed-off-by: David Gibson