Unset copied environment variables in qemuxml2argvtest

Unset copied environment variables in qemuxml2argvtest

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