vdpa: add VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK flag

This feature flag allows the driver enabling virtqueues both before and
after DRIVER_OK.

This is needed for software assisted live migration, so userland can
restore the device status in devices with control virtqueue before the
dataplane is enabled.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Shannon Nelson <shannon.nelson@amd.com>
Message-Id: <20230609092127.170673-2-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>