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

Rather than clear the HV bit from the MSR at guest entry, make it clear
that the hypervisor does not allow the guest to set the bit.

The HV clear is kept in guest entry for now, but a future patch will
warn if it is set.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210412014845.1517916-13-npiggin@gmail.com