virtio-scsi: Set _DRIVER_OK flag before scsi target scanning

Before we start scsi target scanning, we need to set the
VIRTIO_CONFIG_S_DRIVER_OK flag so the device can do setup properly.

This fix a bug when booting tcm_vhost with seabios.

Signed-off-by: Asias He <asias@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 5a7730db57ab0715223421e65b54fb50d6fefe5c)