KVM: do not register i8254 PIO regions until we are initialized

We currently publish the i8254 resources to the pio_bus before the devices
are fully initialized.  Since we hold the pit_lock, its probably not
a real issue.  But lets clean this up anyway.

Reported-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Gregory Haskins <ghaskins@novell.com>
Acked-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Avi Kivity <avi@redhat.com>