tests: report errors in QEMU XML->startup XML tests

Now that the function is only run if requested by
the FLAG_STEAL_VM flag, we know that missing data
is an error, not a request to skip the test.

The existence of the output file is now checked by
virTestCompareToFile, which allows usage of
the VIR_TEST_REGENERATE_OUTPUT=1 env variable
to generate new test cases.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

tests/qemuxml2argvtest.c

@@ -442,18 +442,15 @@ testCompareXMLToStartupXML(const void *data)
     char *actual = NULL;
     int ret = -1;
 
-    if (!info->vm)
-        return EXIT_AM_SKIP;
+    if (!info->vm) {
+        VIR_TEST_DEBUG("VM object missing. Did the args conversion succeed?");
+        return -1;
+    }
 
     if (virAsprintf(&xml, "%s/qemuxml2startupxmloutdata/%s.xml",
                     abs_srcdir, info->name) < 0)
         goto cleanup;
 
-    if (!virFileExists(xml)) {
-        ret = EXIT_AM_SKIP;
-        goto cleanup;
-    }
-
     if (!(actual = virDomainDefFormat(info->vm->def, NULL, format_flags)))
         goto cleanup;