libqos: implement VIRTIO 1.0 FEATURES_OK step

libqos: implement VIRTIO 1.0 FEATURES_OK step

Device initialization has an extra step in VIRTIO 1.0.  The FEATURES_OK
status bit is set to indicate that feature negotiation has completed.
The driver then reads the status register again to check that the device
agrees with the final features.

Implement this step as part of qvirtio_set_features() instead of
introducing a separate function.  This way all existing code works
without modifications.

The check in qvirtio_set_driver_ok() needs to be updated because
FEATURES_OK will be set for VIRTIO 1.0 devices.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20191023100425.12168-9-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Thomas Huth" <thuth@redhat.com>