KVM: PPC: Book3S PR: Always fail transactions in guest privileged state

Currently the kernel doesn't use transaction memory.
And there is an issue for privileged state in the guest that:
tbegin/tsuspend/tresume/tabort TM instructions can impact MSR TM bits
without trapping into the PR host. So following code will lead to a
false mfmsr result:
	tbegin	<- MSR bits update to Transaction active.
	beq 	<- failover handler branch
	mfmsr	<- still read MSR bits from magic page with
		transaction inactive.

It is not an issue for non-privileged guest state since its mfmsr is
not patched with magic page and will always trap into the PR host.

This patch will always fail tbegin attempt for privileged state in the
guest, so that the above issue is prevented. It is benign since
currently (guest) kernel doesn't initiate a transaction.

Test case:
https://github.com/justdoitqd/publicFiles/blob/master/test_tbegin_pr.c

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