vbox: Replace atoi with virStrToLong_i

Parsing is stricter now and doesn't accept trailing characters
after the actual value or non-number strings anymore. atoi just
returns 0 in case it cannot parse a number from the given string.
Now an error is reported for such a string.

src/vbox/vbox_tmpl.c

@@ -598,7 +598,9 @@ static int PRUnicharToInt(PRUnichar *strUtf16) {
     if (!strUtf8)
         return -1;
 
-    ret = atoi(strUtf8);
+    if (virStrToLong_i(strUtf8, NULL, 10, &ret) < 0)
+        ret = -1;
+
     g_pVBoxGlobalData->pFuncs->pfnUtf8Free(strUtf8);
 
     return ret;