Use VBoxManage for all the tasks

$ /usr/bin/VBoxManage createvm --name Debian5 –register //The VM created is an XML file located in your home directory: ~/.VirtualBox/Machines/Debian5/Debian5.xml.

$ /usr/bin/VBoxManage createhd --filename Debian5.vdi --size 4000 --variant Fixed // created under ~/.VirtualBox/HardDisks/Debian.vdi  4GB (4,000MB) in size and with fixed size (non-expanding).

$ /usr/bin/VBoxManage storagectl Debian5 --name "IDE Controller" --add ide --controller PIIX4

$ /usr/bin/VBoxManage storageattach Debian5 --storagectl "IDE Controller" --port 0 --device 0 --type hdd --medium Debian5.vdi

$ /usr/bin/VBoxManage storageattach Debian5 --storagectl "IDE Controller" --port 0 --device 1 --type dvddrive --medium /ISO/debian-40r3-i386-netinst.iso

$ /usr/bin/VBoxManage modifyvm Debian5 --nic1 bridged --cableconnected1 on --bridgeadapter1 eth0

$ /usr/bin/VBoxManage startvm Debian5

http://www.linux-mag.com/id/7673