KVM: PPC: Book3S HV: Ensure MSR[ME] is always set in guest MSR

Rather than add the ME bit to the MSR at guest entry, make it clear
that the hypervisor does not allow the guest to clear the bit.

The ME set is kept in guest entry for now, but a future patch will
warn if it's not present.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Acked-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210412014845.1517916-12-npiggin@gmail.com