KVM: x86: disable paravirt mmu reporting

Disable paravirt MMU capability reporting, so that new (or rebooted)
guests switch to native operation.

Paravirt MMU is a burden to maintain and does not bring significant
advantages compared to shadow anymore.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>