KVM: PPC: Fix compile error in e500_tlb.c

The e500_tlb.c file didn't compile for me due to the following error:

arch/powerpc/kvm/e500_tlb.c: In function ‘kvmppc_e500_shadow_map’:
arch/powerpc/kvm/e500_tlb.c:300: error: format ‘%lx’ expects type ‘long unsigned int’, but argument 2 has type ‘gfn_t’

So let's explicitly cast the argument to make printk happy.

Signed-off-by: Alexander Graf <agraf@suse.de>