kvm: don't try to register to KVM_FAST_MMIO_BUS for non mmio eventfd

kvm: don't try to register to KVM_FAST_MMIO_BUS for non mmio eventfd

commit 8453fecbecae26edb3f278627376caab05d9a88d upstream.

We only want zero length mmio eventfd to be registered on
KVM_FAST_MMIO_BUS. So check this explicitly when arg->len is zero to
make sure this.

Cc: Gleb Natapov <gleb@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>