vduse: add F_QUEUE_READY feature

Add the VDUSE_F_QUEUE_READY feature flag, which tells the kernel to
forward vq ready state changes to the VDUSE userland instance via a
new VDUSE_SET_VQ_READY message.

Without this feature, the VDUSE userland instance has no way to know
when a virtqueue transitions to the ready state and when the driver
starts using the virtqueue.

If the userland instance doesn't support VDUSE_F_QUEUE_READY, the
kernel silently ignores vq ready transitions as before.

If the userland instance negotiates VDUSE_F_QUEUE_READY but fails to
acknowledge a VDUSE_SET_VQ_READY message, the device is marked as
broken.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-ID: <20260415085005.584291-4-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>