KVM: PPC: Enable compatibility mode

The host kernel implements a KVM_REG_PPC_ARCH_COMPAT register which
this uses to enable a compatibility mode if any chosen.

This sets the KVM_REG_PPC_ARCH_COMPAT register in KVM. ppc_set_compat()
signals the caller if the mode cannot be enabled by the host kernel.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[agraf: fix TCG compat setting]
Signed-off-by: Alexander Graf <agraf@suse.de>