KVM: PPC: Destory timer on vcpu destruction

When we destory a vcpu, we should also make sure to kill all pending
timers that could still be up. When not doing this, hrtimers might
dereference null pointers trying to call our code.

This patch fixes spontanious kernel panics seen after closing VMs.

Signed-off-by: Alexander Graf <alex@csgraf.de>
Signed-off-by: Avi Kivity <avi@redhat.com>