When reporting issues regarding virt-manager, virt-install, or any of the other
virt-* tools please run the program using the --debug command line option
(also with --no-fork for virt-manager) 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.

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