esx: Fix esxVI_BuildSelectSet's invalid argument check

The check was altered in 8c48743b
and got too strict, I've no clue how that snuck in. This check
makes every try to open a connection using the ESX driver fail
with an invalid argument error.

Revert the change to the check and add a comment to prevent future
mistakes with this check.

src/esx/esx_vi.c

@@ -1390,7 +1390,11 @@ esxVI_BuildSelectSet(esxVI_SelectionSpec **selectSet,
     esxVI_SelectionSpec *selectionSpec = NULL;
     const char *currentSelectSetName = NULL;
 
-    if (selectSet == NULL || *selectSet != NULL) {
+    if (selectSet == NULL) {
+        /*
+         * Don't check for *selectSet != NULL here because selectSet is a list
+         * and might contain items already. This function appends to selectSet.
+         */
         ESX_VI_ERROR(VIR_ERR_INTERNAL_ERROR, "%s", _("Invalid argument"));
         return -1;
     }