VirtualBox
Quick start
List virtual machines with detail
$ VBoxManage list vms -l
Start a virtual machine
$ VboxManage startvm "instance_name"
These are self explanatory instance commands
$ VBoxManage controlvm "instance_name" pause
$ VBoxManage controlvm "instance_name" resume
$ VBoxManage controlvm "instance_name" reset
$ VBoxManage controlvm "instance_name" poweroff
$ VBoxManage controlvm "instance_name" savestate
Create a new virtual machine with default options
$ VboxManage create -name "instance_name" -register
Change settings of a virtual machine
$ VBoxManage modifyvm "instance_name" -memory "1024MB"
References
http://www.virtualbox.org/manual/ch08.html#vboxmanage-modifyvm
http://www.go2linux.org/linux/2011/01/vboxmanage-control-and-manage-virtualbox-command-line-890?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+Go2linux+(Linux+operating+system+with+Go2Linux)