virtio_net: rx remove premapped failover code

Now, the premapped mode can be enabled unconditionally.

So we can remove the failover code for merge and small mode.

The virtnet_rq_xxx() helper would be only used if the mode is using pre
mapping. A check is added to prevent misusing of these API.

Tested-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>