hw/virtio: fix Power Management Control Register for PCI Express virtio devices

Make Power Management State flag writable to conform
with the PCI Express spec.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>