booting qemu/kvm directly from a file system

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http://unix.stackexchange.com – Having used Xen and its pygrub for a while, I'm now trying to do something similar with KVM/qemu. Basically, I have a LVM logical volume (though that could as well be a file) that contains the root filesystem of a Linux distribution (no partitioning, just FS directly on disk) which also contains a /boot/grub/grub.cfg, and I'd like to be able to boot that. I was under the impression that the grub-firmware-qemu package on Debian/Ubuntu was intended for that, but I have not managed to make it work. AFAICT, the grub.bin image contains a hardcoded memdisk with a grub.cfg file with a few menu opti (HowTos)