When running with -M isapc, there is no env->apic_state. Fix
cpu_get/set_apic_* helpers to handle this corner case gracefully.

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@7048 c046a42c-6fe2-441c-8c8c-71466251a162