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

tests/qemuxml2argvtest.c

@@ -184,6 +184,9 @@ mymain(int argc, char **argv)
 #define DO_TEST(name, extraFlags)                       \
         DO_TEST_FULL(name, extraFlags, NULL)
 
+    /* Unset or set all envvars here that are copied in qemudBuildCommandLine
+     * using ADD_ENV_COPY, otherwise these tests may fail due to unexpected
+     * values for these envvars */
     setenv("PATH", "/bin", 1);
     setenv("USER", "test", 1);
     setenv("LOGNAME", "test", 1);
@@ -191,6 +194,8 @@ mymain(int argc, char **argv)
     unsetenv("TMPDIR");
     unsetenv("LD_PRELOAD");
     unsetenv("LD_LIBRARY_PATH");
+    unsetenv("QEMU_AUDIO_DRV");
+    unsetenv("SDL_AUDIODRIVER");
 
     DO_TEST("minimal", QEMUD_CMD_FLAG_NAME);
     DO_TEST("machine-aliases1", 0);