KVM: PPC: Book3S PR: Avoid changing TS bits when exiting guest

PR KVM host usually runs with TM enabled in its host MSR value, and
with non-transactional TS value.

When a guest with TM active traps into PR KVM host, the rfid at the
tail of kvmppc_interrupt_pr() will try to switch TS bits from
S0 (Suspended & TM disabled) to N1 (Non-transactional & TM enabled).

That will leads to TM Bad Thing interrupt.

This patch manually sets target TS bits unchanged to avoid this
exception.

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