dataplane: support viostor virtio-pci status bit setting

The viostor virtio-blk driver for Windows does not use the
VIRTIO_CONFIG_S_DRIVER bit.  It only sets the VIRTIO_CONFIG_S_DRIVER_OK
bit.

The viostor driver refreshes the virtio-pci status byte sometimes while
the guest is running.  We misinterpret 0x4 (VIRTIO_CONFIG_S_DRIVER_OK)
as an indication that virtio-blk-data-plane should be stopped since 0x2
(VIRTIO_CONFIG_S_DRIVER) is missing.  The result is that the device
becomes unresponsive.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>