Check if the i8254 timer is active before deactivating it (Alexander Graf)

Check if the i8254 timer is active before deactivating it (Alexander Graf)

The HPET emulation can disable the i8254 when the HPET is
in legacy mode, thus emulating the i8254's behavior.

But if it does, the i8254 doesn't have to be running, so
let's check to see if the timer works and not disable it
if it's not.

This fixes a segmentation fault when running Mac OS X as
guest os.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6460 c046a42c-6fe2-441c-8c8c-71466251a162