KVM: PPC: Move memslot memory allocation into prepare_memory_region()

Allocate the rmap array during kvm_arch_prepare_memory_region() to pave
the way for removing kvm_arch_create_memslot() altogether.  Moving PPC's
memory allocation only changes the order of kernel memory allocations
between PPC and common KVM code.

No functional change intended.

Acked-by: Paul Mackerras <paulus@ozlabs.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>