Ah, I didn't catch it before - you're running from the Virtual Machine Manager. If you ticked the option for "kernel / hardware acceleration" that means you're running KVM (qemu-kvm). Or you were, when it was faster. You should check for /dev/kvm to see if this kernel module is still being loaded, as it sounds like maybe it isn't.
I played around with the VMM briefly one afternoon recently, and couldn't see where you can switch this setting on / off after the fact, but maybe there is a way to do it from the shell - see here
for more information about managing VMM from the shell.
Alternatively, you could try to run KVM directly from the shell without messing about with the VMM. It allows you to specify lots of options, and you'll also get to see the output of KVM which might tell you what's going wrong. Run KVM just like QEMU
, except specify qemu-kvm
instead of qemu
qemu-kvm -m 512 -hda /path/to/your.img