When dom0 do not provide the kernel, it should also not set kernel
command line in libvirt config. Otherwise qemu in stubdom fails to start
because it get -append option without -kernel, which is illegal
configuration.
FixesQubesOS/qubes-issues#3339
Unfortunately, python3-gi is unavailable in travis-ci environment (and
installing it from repository doesn't help, because system python is not
used). Instead, add mockup of pydbus module - to silence ImportError.
Tests will cover it with unittest.mock anyway.
