UPSTREAM: virtio_config: introduce a new .enable_cbs method

This patch introduces a new method to enable the callbacks for config
and virtqueues. This will be used for making sure the virtqueue
callbacks are only enabled after virtio_device_ready() if transport
implements this method.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20211019070152.8236-4-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit d50497eb)
Bug: 196772804
Signed-off-by: Keir Fraser <keirf@google.com>
Change-Id: I17ea164aa100d690ebde3b2f6c2e5514a9b5cfd9