Creating a bootable qemu image from a root filesystem and bootloader
Create a hard disk image
qemu-img create -f qcow G
Use a live CD (better to have a lighter OS like system rescue CD to make the process faster) and the image created to boot.
qemu -cdrom /dev/cdrom -hda -boot d
Once system is booted use a partition editing tool (like fdisk, cfdisk, parted, gparted, qtparted ...) to partition the image.
create the necessary partitions (root and swap partitions boot, home ... if required)