qemu_timedate_diff() shouldn't modify its argument.

The caller of qemu_timedate_diff() does not expect that tm it passes to
the function will be modified, but mktime() is destructive and modifies
its argument. Pass a copy of tm to it and set tm_isdst so that mktime()
will not rely on it since its value may be outdated.
Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

vl.c

@@ -460,8 +460,11 @@ int qemu_timedate_diff(struct tm *tm)
     if (rtc_date_offset == -1)
         if (rtc_utc)
             seconds = mktimegm(tm);
-        else
-            seconds = mktime(tm);
+        else {
+            struct tm tmp = *tm;
+            tmp.tm_isdst = -1; /* use timezone to figure it out */
+            seconds = mktime(&tmp);
+	}
     else
         seconds = mktimegm(tm) + rtc_date_offset;