Description of problem:

In the wizard for creating a new virtual machine, there is a check in create.py
that determines whether or not to show the Paravirtualization/Full
Virtualization screen based on the value of xeninst.util.is_hvm_capable(). 
However, this logic is only implemented for the Forward button, not for Back. 
The end result is that on machines that are not HVM capable, hitting Back on the
Installation Media screen always shows the virtualization selection.


Version-Release number of selected component (if applicable):

virt-manager-0.2.2-1.fc6


Steps to Reproduce:
1. Start the new VM wizard on a machine that does not support HVM
2. On the Installation Media screen, click Back.

  
Actual results:

The Paravirtualization/Full Virtualization selection screen is shown.


Expected results:

The logical previous screen (machine name in the current codebase) should be
shown.  The virtualization type was skipped going forward, it should be skipped
going backwards as well.


Additional info:

I have attached a patch that does a simple check of
xeninst.util.is_hvm_capable() in the back code for screen 4 (Installation
Media).  If true, show the selection, else show the machine name screen.