Make it easy to retrieve DeviceInfo object out of DeviceAssignment
object. The only missing piece of information for that is device class,
so add it. Make it optional, as it can be filled on demand when passing
the object through DeviceCollection (either by listing devices, or
attaching/detaching).
This is mostly to ease handling options in libvirt template - to get
them, you need to use `assignments()`, istead of `persistent()` or
`attached()`, but there were no _simple_ way of getting actual device
object.
This also makes DeviceCollection._device method not needed anymore.
d0b8933374