KVM: PPC: Don't always report hash MMU capability for P9 < DD2.2

These machines don't support running both MMU types at the same time,
so remove the KVM_CAP_PPC_MMU_HASH_V3 capability when the host is
using Radix MMU.

[paulus@ozlabs.org - added defensive check on
 kvmppc_hv_ops->hash_v3_possible]

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>