Use CompizConfig Settings Manager GUI. Usability of Windows 7 in VirtualBox, every time I pressed Windows Key Button on my keyboard or tried to Alt + TAB inside a VirtualBox and Ubuntu was a host machine, a Dash Home appeared even when I was using virtual Windows.
To install and run CompizConfit Settings Manager
$ sudo apt-get install compizconfig-settings-manager
Go to Ubuntu Unity Plugin > Behavior tab and make Key to show the launcher Disabled.
Notice, that I did not use sudo to run ccsm (Compiz Config Settings Manager), if you do it with sudo, it will turn the key off only for root, so launch ccsm from terminal or dash home using a username that you want to alter the settings for.
Disable Key to show the HUD, by clicking the shortcut button and un-checking Enabled, and you will get rid of annoying popup rectangle over the Dash Home with text Type your command every time when Alt is pressed, you may also decide to choose different shortcut key.