Currently, I am able to reproduce this problem only when I try to run a virtual machine in a virtual machine. This problem appears also with libvirt.
Here after are used commands:
/usr/bin/qemu-system-ppc64 -name /root -enable-kvm -M pseries -smp 4 -m 4G -nographic -nodefaults -monitor stdio -serial pty -device virtio-net-pci,netdev=net10051,mac=c0:ff:ee:00:00:51 -netdev bridge,br=br0,id=net10051 -hda /root/hd.qcow2 --cdrom /root/Fedora-Server-dvd-ppc64le-25-Alpha-1.2.iso -S

virt-install --machine pseries --name f25_ppc64le_alpha1.2 --hvm --virt-type kvm --arch ppc64le --boot cdrom,hd --network bridge=virbr0,model=virtio-net-pci,mac=c0:ff:ee:00:00:51 --disk path=/var/lib/libvirt/images/hd.qcow2,size=10,format=qcow2,bus=virtio,cache=none --memory 4096 --vcpus sockets=1,cores=1,threads=1 --cdrom /var/lib/libvirt/images/Fedora-Server-dvd-ppc64le-25-Alpha-1.2.iso

Note: when I try these commands in a host machine (f25 ppc64le) on a baremetal, I hit the following problem:
qemu-system-ppc64: -device virtio-net-pci,netdev=net10051,mac=c0:ff:ee:00:00:51: failed to find romfile "efi-virtio.rom"

(In reply to Menanteau Guy from comment #0)
> I am trying to run last f25 ppc64le qemu
> (qemu-system-ppc-2.7.0-0.2.rc3.fc25) and it failed with:
> 
> qemu: fatal: Unknown MMU model
> 

I too saw similar issue in different scenario and found a patch at https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg04879.html. I did a scratch build with mentioned patch which is available at http://ppc.koji.fedoraproject.org/koji/taskinfo?taskID=3753334 .
Can you please try updating qemu with sub-packages available in this scratch build and see if it fixes the problem? Worked for me.

Hi, I tested your qemu (got form your koji link) in same environment as I got the problem and it works fine with your fix. Thanks

This problem is still present on fedora 25 Beta 1.2 release with qemu-2.7.0-1

f25 Beta-1.2 ppc64le:
http://ppc.koji.fedoraproject.org/compose/25/Fedora-25-20161010.0/compose/Server/ppc64le/iso/Fedora-Server-dvd-ppc64le-25_Beta-1.2.iso

dwg, did that patch fall of your radar? I don't see it in qemu.git yet

https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg04879.html

It looks like an alternative patch got pulled in in it's place for 2.8:

[PATCH] 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 4322e8ced5aaac719
("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>

Thanks Michael! That's commit 0d594f5565837fe2886a8aa307ef8abb65eab8f7 , I'll pull it in on the next build

qemu-2.7.0-6.fc25 has been pushed to the Fedora 25 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2016-190e517826

qemu-2.7.0-6.fc25 has been pushed to the Fedora 25 stable repository. If problems still persist, please make note of it in this bug report.