log: do not log if QEMU is daemonized but without -D

Commit 96c33a45 ("log: Redirect stderr to logfile if deamonized",
2016-02-22) wanted to move stderr of a daemonized QEMU to the file
specified with -D.

However, if -D was not passed, the patch had the side effect of not
redirecting stderr to /dev/null.  This happened because qemu_logfile
was set to stderr rather than the expected value of NULL.  The fix
is simply in the "if" condition of do_qemu_set_log; the "if" for
closing the file is also changed to match.
Reported-by: NJan Tomko <jtomko@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

util/log.c

@@ -56,7 +56,8 @@ void do_qemu_set_log(int log_flags, bool use_own_buffers)
 #ifdef CONFIG_TRACE_LOG
     qemu_loglevel |= LOG_TRACE;
 #endif
-    if ((qemu_loglevel || is_daemonized()) && !qemu_logfile) {
+    if (!qemu_logfile &&
+        (is_daemonized() ? logfilename != NULL : qemu_loglevel)) {
         if (logfilename) {
             qemu_logfile = fopen(logfilename, log_append ? "a" : "w");
             if (!qemu_logfile) {
@@ -72,6 +73,7 @@ void do_qemu_set_log(int log_flags, bool use_own_buffers)
             }
         } else {
             /* Default to stderr if no log file specified */
+            assert(!is_daemonized());
             qemu_logfile = stderr;
         }
         /* must avoid mmap() usage of glibc by setting a buffer "by hand" */
@@ -89,7 +91,8 @@ void do_qemu_set_log(int log_flags, bool use_own_buffers)
             log_append = 1;
         }
     }
-    if (!qemu_loglevel && !is_daemonized() && qemu_logfile) {
+    if (qemu_logfile &&
+        (is_daemonized() ? logfilename == NULL : !qemu_loglevel)) {
         qemu_log_close();
     }
 }