Do not crash if the other object is completely different type. Return
False ("unequal") instead.
This crashed preparing list of devices in qubes-vm-boot-from-device.
FixesQubesOS/qubes-issues#3182
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 also makes DeviceCollection._device method not needed anymore.
