virt-manager should prefill the "Install Media URL" field with a sensible repo.
The 0.2.5 release of virt-manager will have URL history in this field so it
remembers the last 10 urls used for installs. This will be pushed out as a FC6
update in the reasonably near future.
Longer term we need to consider whether we can pre-populate download URLs for
common paravirt OS', however, we don't want to link to particular servers & we
don't have support for dealing with the Fedora mirror system yet.
The 0.2.5 release or later is now available in FC6 which remembers install media