There are a couple of things we can still try.
Do you have synaptic installed? Install this graphical front-end then try searching and installing the packages from there. Either that or try using: sudo aptitude install virt-manager
If it doesn't work, try:
sudo nano edit /etc/apt/sources.list
Delete everything in there and paste these lines with CTRL+SHIFT+V:
deb http://ftp.us.debian.org/debian jessie main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free
exit with CTRL+X
save with CTRL+M
and run apt-get update as root again then see if it works.