virtio-blk: use aio handler for data plane

virtio-blk: use aio handler for data plane

In addition to handling IO in vcpu thread and in io thread, dataplane
introduces yet another mode: handling it by AioContext.

This reuses the same handler as previous modes, which triggers races as
these were not designed to be reentrant.  Use a separate handler just
for aio, and disable regular handlers when dataplane is active.

Signed-off-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>