x86: Fix powerdown for non-ACPI case

Trivial fix for a corner case: system_shutdown on isapc machines causes
qemu to segfaults due to accessing the uninitialized pm_state. Issue a
system shutdown instead.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5130 c046a42c-6fe2-441c-8c8c-71466251a162
