tests/vhost-user-bridge: workaround stale vring base

tests/vhost-user-bridge: workaround stale vring base

This patch is a similar solution to what Yuanhan Liu/Huawei Xie have
suggested for DPDK. When vubr quits (killed or crashed), a restart of
vubr would get stale vring base from QEMU. That would break the kernel
virtio net completely, making it non-work any more, unless a driver
reset is done.

So, instead of getting the stale vring base from QEMU, Huawei suggested
we could get a proper one from used->idx. This works because the queues
packets are processed in order.

Signed-off-by: Marc-André Lureau <[email protected]>
Tested-by: Yuanhan Liu <[email protected]>
Reviewed-by: Yuanhan Liu <[email protected]>
Reviewed-by: Victor Kaplansky <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>