vhost-user: Support vhost_dev_start

vhost-user: Support vhost_dev_start

The motivation of adding vhost-user vhost_dev_start support is to
improve backend configuration speed and reduce live migration VM
downtime.

Today VQ configuration is issued one by one. For virtio net with
multi-queue support, backend needs to update RSS (Receive side
scaling) on every rx queue enable. Updating RSS is time-consuming
(typical time like 7ms).

Implement already defined vhost status and message in the vhost
specification [1].
(a) VHOST_USER_PROTOCOL_F_STATUS
(b) VHOST_USER_SET_STATUS
(c) VHOST_USER_GET_STATUS

Send message VHOST_USER_SET_STATUS with VIRTIO_CONFIG_S_DRIVER_OK for
device start and reset(0) for device stop.

On reception of the DRIVER_OK message, backend can apply the needed setting
only once (instead of incremental) and also utilize parallelism on enabling
queues.

This improves QEMU's live migration downtime with vhost user backend
implementation by great margin, specially for the large number of VQs of 64
from 800 msec to 250 msec.

[1] https://qemu-project.gitlab.io/qemu/interop/vhost-user.html

Signed-off-by: Yajun Wu <yajunw@nvidia.com>
Acked-by: Parav Pandit <parav@nvidia.com>
Message-Id: <20221017064452.1226514-3-yajunw@nvidia.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>