This was/is a key part of the Red Hat Virtualization product that was acquired from Qumranet. I haven't used it for many years, but getting a working client was difficult back in the day, especially if you were on Mac. I always found NX Windows was better for X and RDP was better for windows servers, but interested to see the technology is still around.
Spice agent is the equivalent of vmware/virtualbox "guest additions," and is the thing you want to install on your guest if you're using virt-manager on Linux.<p>It lets you do things like dynamic resizing, copy/paste, file sharing, etc.