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 <[e-mail]>
Acked-by: Shannon Nelson <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

include/uapi/linux/vhost_types.h

···
 #define VHOST_BACKEND_F_SUSPEND  0x4
 /* Device can be resumed */
 #define VHOST_BACKEND_F_RESUME  0x5
+/* Device supports the driver enabling virtqueues both before and after
+ * DRIVER_OK
+ */
+#define VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK  0x6
 
 #endif