KVM: docs: mmu: KVM support exposing SLAT to guests

Fix outdated statement that KVM is not able to expose SLAT
(Second-Layer-Address-Translation) to guests.
This was implemented a long time ago...

Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>