qemu: command: Truncate the chardev logging file even if append is not present

Our documentation states that the chardev logging file is truncated
unless append='on' is specified. QEMU also behaves the same way and
truncates the file unless we provide the argument. The new virlogd
implementation did not honor if the argument was missing and continued
to append to the file.

Truncate the file even when the 'append' attribute is not present to
behave the same with both implementations and adhere to the docs.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1420205

src/qemu/qemu_command.c

@@ -4827,7 +4827,8 @@ qemuBuildChrChardevFileStr(virLogManagerPtr logManager,
         int flags = 0;
         int logfd;
 
-        if (appendval == VIR_TRISTATE_SWITCH_OFF)
+        if (appendval == VIR_TRISTATE_SWITCH_ABSENT ||
+            appendval == VIR_TRISTATE_SWITCH_OFF)
             flags |= VIR_LOG_MANAGER_PROTOCOL_DOMAIN_OPEN_LOG_FILE_TRUNCATE;
 
         if ((logfd = virLogManagerDomainOpenLogFile(logManager,