To create virtual image with 20GB space: $ qemu-img create ubuntu.img 20G

To boot created image with iso and 512mb ram $ qemu-system-i386 -drive format=raw,media=cdrom,file=ubuntu.img -cdrom ../Downloads/mini.iso -m 1024

To boot image with 1GB ram: $ qemu-system-i386 -hda ubuntu.img -m 1024