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.
Since we use qubes.VMShell service now and send requested command on its
stdin, we need to terminate that shell after requested command -
otherwise the service will not terminate automatically waiting for
further input (next commands).
qubesd_call expect "bytes" type. Additionally serialize false value as
empty string (which is treated by python as false value), because
otherwise would be serialized to (non-empty) string, which is true value
in python.
