KVM: ppc: e500: Fix the bug that KVM is unstable in SMP

TLB entry should enable memory coherence in SMP.

And like commit 631fba9dd3aca519355322cef035730609e91593,
remove guard attribute to enable the prefetch of guest memory.

Signed-off-by: Liu Yu <yu.liu@freescale.com>
Signed-off-by: Avi Kivity <avi@redhat.com>