Had a little trouble accessing virt-manager or firefox over SSH with X11 forwarding from my headless Ubuntu 16.04 KVM server to my Windows 7 work-laptop with Cygwin.
It kept giving me this error:
Couldn't open libGL.so.1: libGL.so.1: cannot open shared object file: No such file or directory
Installing libglu1-mesa fixed it for me.
apt-get install libglu1-mesa -y