The test expected all environment variables copied in qemudBuildCommandLine
to have known values. So all of them have to be either set to a known value
or be unset. SDL_VIDEODRIVER and QEMU_AUDIO_DRV are not handled at all but
should be handled. Unset both, otherwise the test will fail if they are set
in the testing environment.

* src/qemu/qemu_conf.c: add a comment about copied environment variables
  and qemuxml2argvtest
* tests/qemuxml2argvtest.c: unset SDL_VIDEODRIVER and QEMU_AUDIO_DRV