apt-get install qemu-kvm qemu virt-manager virt-viewer libvirt-bin
qemu-img create ubuntu.img 10G
qemu-img create -f qcow2 ubuntu.qcow 10G
qemu-system-x86_64 -hda ubuntu.img -boot d -cdrom /ubuntu-15.04-server-amd64.iso -m 512
qemu -hda ubuntu.img -m 512

Running Openwrt with QEMU needs some files from openwrt trunk, get them from here

qemu-system-arm -M realview-pbx-a9 -m 1024M -nographic \
    -kernel openwrt-realview-vmlinux.elf \
    -sd openwrt-realview-sdcard.img \
    -append "console=ttyAMA0 verbose debug root=/dev/mmcblk0p1"