Description of problem:
The chosen file size for storage image is reset back to default (4000 MB) when
going back in virt-manager UI.

Version-Release number of selected component (if applicable):
virt-manager-0.5.3-8.el5

How reproducible:
Always

Steps to Reproduce:
1. Run virt-manager and create a new guest (FV in my case)
2. At "Assigning storage space" select "Simple file" and change the default size
3. Proceed to the next screen (or next 2 screens)
4. Go back at the "Assigning storage space" screen
  
Actual results:
File size is reset to default (4000 MB)

Expected results:
File size is the same as entered by the user

Additional info:
I assume that this is caused by design, i.e. when the screen shows up the UI
fills in a default value for the file size (which in this case could be
overwriting the previous value). This is causing problems with usability for the
end user. When one configures a guest and goes Back/Forward to modify settings
one expects that everything they do not modify stays the same. In particular
this has caused a few "not enough disk space" errors for me.