When reporting issues regarding virt-manager please run the program using the
--debug --no-fork command line options and provide the output.

Often the problem isn't related to virt-manager at all. Please also check the
libvirt and VM logfiles and try to reproduce with virsh or virt-install.

http://wiki.debian.org/libvirt/Debugging has more information on howto provide
debugging information for the virt-* tools.