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.
