Replace remaining gmtime, localtime by gmtime_r, localtime_r

This allows removing of MinGW specific code and improves
reentrancy for POSIX hosts.

[Removed unused ret variable in qemu_get_timedate() to fix warning:
vl.c: In function ‘qemu_get_timedate’:
vl.c:451:16: error: variable ‘ret’ set but not used [-Werror=unused-but-set-variable]
-- Stefan Hajnoczi]

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/vl.c b/vl.c
index f056c95..aed4182 100644
--- a/vl.c
+++ b/vl.c
@@ -448,21 +448,18 @@
 void qemu_get_timedate(struct tm *tm, int offset)
 {
     time_t ti;
-    struct tm *ret;
 
     time(&ti);
     ti += offset;
     if (rtc_date_offset == -1) {
         if (rtc_utc)
-            ret = gmtime(&ti);
+            gmtime_r(&ti, tm);
         else
-            ret = localtime(&ti);
+            localtime_r(&ti, tm);
     } else {
         ti -= rtc_date_offset;
-        ret = gmtime(&ti);
+        gmtime_r(&ti, tm);
     }
-
-    memcpy(tm, ret, sizeof(struct tm));
 }
 
 int qemu_timedate_diff(struct tm *tm)