ppc/kvm: Mark 64kB page size support as disabled if not available

QEMU currently refuses to start with KVM-PR and only prints out

	qemu: fatal: Unknown MMU model 851972

when being started there. This is because commit 4322e8ce
("ppc: Fix 64K pages support in full emulation") introduced a new
POWERPC_MMU_64K bit to indicate support for this page size, but
it never gets cleared on KVM-PR if the host kernel does not support
this. Thus we've got to turn off this bit in the mmu_model for KVM-PR.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit 0d594f55)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>