vhost: multiqueue support

This patch lets vhost support multiqueue. The idea is simple, just launching
multiple threads of vhost and let each of vhost thread processing a subset of
the virtqueues of the device. After this change each emulated device can have
multiple vhost threads as its backend.

To do this, a virtqueue index were introduced to record to first virtqueue that
will be handled by this vhost_net device. Based on this and nvqs, vhost could
calculate its relative index to setup vhost_net device.

Since we may have many vhost/net devices for a virtio-net device. The setting of
guest notifiers were moved out of the starting/stopping of a specific vhost
thread. The vhost_net_{start|stop}() were renamed to
vhost_net_{start|stop}_one(), and a new vhost_net_{start|stop}() were introduced
to configure the guest notifiers and start/stop all vhost/vhost_net devices.

Signed-off-by: Jason Wang <jasowang@redhat.com>
virtio: introduce virtio_del_queue()

Some device (such as virtio-net) needs the ability to destroy or re-order the
virtqueues, this patch adds a helper to do this.

Signed-off-by: Jason Wang <jasowang>
virtio: add a queue_index to VirtQueue

Add a queue_index to VirtQueue and a helper to fetch it, this could be used by
multiqueue supported device.

Signed-off-by: Jason Wang <jasowang@redhat.com>
virtio-net: separate virtqueue from VirtIONet

To support multiqueue virtio-net, the first step is to separate the virtqueue
related fields from VirtIONet to a new structure VirtIONetQueue. The following
patches will add an array of VirtIONetQueue to VirtIONet based on this patch.

Signed-off-by: Jason Wang <jasowang@redhat.com>
virtio-net: multiqueue support

This patch implements both userspace and vhost support for multiple queue
virtio-net (VIRTIO_NET_F_MQ). This is done by introducing an array of
VirtIONetQueue to VirtIONet.

Signed-off-by: Jason Wang <jasowang@redhat.com>
virtio-net: migration support for multiqueue

This patch add migration support for multiqueue virtio-net. Instead of bumping
the version, we conditionally send the info of multiqueue only when the device
support more than one queue to maintain the backward compatibility.

Signed-off-by: Jason Wang <jasowang@redhat.com>
virtio-net: compat multiqueue support

Disable multiqueue support for pre 1.4.

Signed-off-by: Jason Wang <jasowang@redhat.com>