#8 Fix Create VM wizard

[chengsun]

The Create VM wizard needs a bit of TLC.

* Back functionality is broken.
* Setting initial vCPU and memory values does not work.
* The title never updates from "Select an OS for this VM"
* Wizard is dependent on the host which is selected in the main tree. If you click on a different host, then it tries to the VM on that host instead. It should store this in its own variable.
* etc.

The wizard needs to be moved into its own class, and the dialog should be made modal.