In GNOME you can set the toolbar button style (icons only, text only, text below or beside icons). VMM should respect this setting. Currently, it always uses text below icons, which takes away a lot of screen space.
Thanks for the report! This is now fixed upstream: http://hg.et.redhat.com/virt/applications/virt-manager--devel?cs=fb4cafd8c490 Closing as UPSTREAM.