Create disk:
qemu-img create -f qcow2 disk_name.img 5G
Launch VM from install medium:
qemu-system-x86_64 -hda disk_name.img -cdrom /path/to/iso_image.iso -m 1024 -boot d
Boot from hard drive:
qemu-system-x86_64 disk_name.img -m 1024