KVM: PPC: Move guest enter/exit down into subarch-specific code

Instead of doing the kvm_guest_enter/exit() and local_irq_dis/enable()
calls in powerpc.c, this moves them down into the subarch-specific
book3s_pr.c and booke.c.  This eliminates an extra local_irq_enable()
call in book3s_pr.c, and will be needed for when we do SMT4 guest
support in the book3s hypervisor mode code.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>