From the command-line there's been support for the open-source Linux virtualization stack for handling snapshots when the VM is using QCOW2, but it hasn't been supported by a graphical user-interface with virt-manager. Having this easy ability to add/delete/list/run VM snapshots from a UI has been a long sought feature by open-source Linux desktop virtualization users -- similar support has been there within VMware and VirtualBox for ages. A VM snapshot is the state and data of a virtual machine captured at a specific time.
With this virt-manager Git commit, "experimental" support is now in place for dealing with virtual machine snapshots. The UI is in place but current limitations are only internal snapshots are supported right now and QCOW2 snapshots are the simpliest to manage and very mature. External snapshots are supported using recent QEMU and libvirt code, but it's harder to manage from virt-manager.
This virt-manager UI snapshot is one of the features of Fedora 20 and was done by Red Hat's Cole Robinson.