KVM: PPC: Book3S PR: Turn on FP/VSX/VMX MSR bits in kvmppc_save_tm()

kvmppc_save_tm() invokes store_fp_state/store_vr_state(). So it is
mandatory to turn on FP/VSX/VMX MSR bits for its execution, just
like what kvmppc_restore_tm() did.

Previously HV KVM has turned the bits on outside of function
kvmppc_save_tm().  Now we include this bit change in kvmppc_save_tm()
so that the logic is cleaner. And PR KVM can reuse it later.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Reviewed-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>