The 512 ram default for new VMs in virt-manager is too low for f15 guests, causing crashes during the install process: https://bugzilla.redhat.com/show_bug.cgi?id=682555#c46 Probably best to bump the default to 1GB in the short term. Long term plan has always been to have ram defaults depend on the chosen OS, but it's a bit late for that in F15
This is already in F15
virt-manager-0.8.7-6.fc15 has been submitted as an update for Fedora 15. https://admin.fedoraproject.org/updates/virt-manager-0.8.7-6.fc15
virt-manager-0.8.7-6.fc15 has been pushed to the Fedora 15 stable repository. If problems still persist, please make note of it in this bug report.