Fix Create VM wizard

[TwoWheelDev]

From: https://sourceforge.net/p/openxenmanager/bugs/8/ 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.