VBoxManage bash completion script.

This version of the completion was initially based on Sebastian T. Hafner
script. However, in some point of time I've decided to rewrite it almost from
scratch.

Current version of script was written and tested against VBoxManage in version
4.3.18, and supports commands:

- adoptstate
- bandwidthctl
- clonehd
- clonevm
- closemedium
- controlvm
- convertfromraw
- createhd
- createvm
- debugvm
- list
- showhdinfo
- showvminfo
- snapshot
- startvm
- storageattach
- storagectl
- unregistervm
- usbfilter