Description of problem:
Trying to using xenguest-install with "rescue" generates a traceback from anaconda.



Version-Release number of selected component (if applicable):
anaconda-11.1.0.97-1

How reproducible:
100%

Steps to Reproduce:
1. xenguest-install -n domu -r 512 -f /root/domu.img -s 4 --nographics -p -l
http://hummer.zko.hp.com/fedora/linux/core/development/ia64/os/ -x 'ide0=noprobe
ide1=noprobe ide2=noprobe ide3=noprobe rescue nomount'

You could probably reproduce it simply booting a Rawhide CD with "rescue" since
the bug seems to be pretty apparent in the anaconda code.
  
Actual results:
    Running anaconda, the Fedora Core rescue mode - please wait...
    Traceback (most recent call last):
      File "/usr/bin/anaconda", line 703, in ?
        rescue.runRescue(anaconda)
      File "/usr/lib/anaconda/rescue.py", line 260, in runRescue
        runShell(screen)
    UnboundLocalError: local variable 'screen' referenced before assignment
    install exited abnormally [1/1]

Expected results:
Rescue shell