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: Michael Avdienko <whitearchey@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>