Red Hat Upgrade Tool fails when loop mounting a physical device

Issue

• When attempting to use the Red Hat Upgrade Tool with the --device option on a loop mounted device.

• This causes the upgrade tool to return the following python backtrace.

HOOK-pkgdowngrades: INFO: done
Traceback (most recent call last):
  File "/usr/bin/redhat-upgrade-tool", line 432, in <module>
    main(args)
  File "/usr/bin/redhat-upgrade-tool", line 372, in main
    setup_media_mount(args.device, args.iso)
  File "/usr/lib/python2.6/site-packages/redhat_upgrade_tool/sysprep.py", line 124, in setup_media_mount
    unit = write_prep_mount(mediamnt, upgrade_prep_dir, iso)
  File "/usr/lib/python2.6/site-packages/redhat_upgrade_tool/media.py", line 137, in write_prep_mount
    mount = fix_loop_entry(mount, iso)
  File "/usr/lib/python2.6/site-packages/redhat_upgrade_tool/media.py", line 94, in fix_loop_entry
    return mnt._replace(dev=realpath(iso), opts=opts)
  File "/usr/lib64/python2.6/posixpath.py", line 357, in realpath
    if isabs(filename):
  File "/usr/lib64/python2.6/posixpath.py", line 52, in isabs
    return s.startswith('/')
AttributeError: 'NoneType' object has no attribute 'startswith'