qga: Fix shutdown command of guest agent to work with SysV

For now guest agent uses following command to shutdown system:
shutdown -P +0 "blabla"
but this syntax works only with shutdown command from systemd or upstart,
because SysV shutdown requires -h switch.

Following patch changes the command so it works with systemd, upstart and SysV

With upstart/systemd qga use one of thee commands, depending on 'mode' parameter:
  shutdown -P +0 "..."
  shutdown -H +0 "..."
  shutdown -r +0 "..."
SysV equivalents for these are:
  shutdown -h -P +0 "..."
  shutdown -h -H +0 "..."
  shutdown -h -r +0 "..."
and these retain their meaning with upstart/systemd.

According to FreeBSD manpages, shutdown does not accept -P and -H options. Commands should be:
  shutdown -p +0 "..."
  shutdown -h +0 "..."
  shutdown -r +0 "..."

shutdown in Solaris does not accept any of -hHpPr and does not accept time in "+0" format
Signed-off-by: NMichael Avdienko <whitearchey@gmail.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qga/commands-posix.c

@@ -99,7 +99,7 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, Error **err)
         reopen_fd_to_null(1);
         reopen_fd_to_null(2);
 
-        execle("/sbin/shutdown", "shutdown", shutdown_flag, "+0",
+        execle("/sbin/shutdown", "shutdown", "-h", shutdown_flag, "+0",
                "hypervisor initiated shutdown", (char*)NULL, environ);
         _exit(EXIT_FAILURE);
     } else if (pid < 0) {