When trying to connect to a KVM using Virtual Machine Manager on Linux Mint 19, I receive the error

Error connecting to graphical console. Error opening Spice console, SpiceClientGtk missing.

This is because the host machine is missing the spice GTK client. In ubuntu 19 it’s easy enough to install using apt

sudo apt install gir1.2-spiceclientgtk-3.0

On older Linux Mint installations (18) you may need to run this instead:

sudo apt-get install gir1.2-spice-client-gtk-3.0