vhost: improve device readiness notifications

vhost: improve device readiness notifications

Some guest drivers may not configure disabled virtio queues.

In this case, the vhost management never notifies the application and
the vDPA device readiness because it waits to the device to be ready.

The current ready state means that all the virtio queues should be
configured regardless the enablement status.

In order to support this case, this patch changes the ready state:
The device is ready when at least 1 queue pair is configured and
enabled.

So, now, the application and vDPA driver are notifies when the first
queue pair is configured and enabled.

Also the queue notifications will be triggered according to the new
ready definition.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>