app-emulation/virt-manager (at least version 0.9.4 - the latest currently in portage, didn't test previous versions) can successfully build and run with the spice flag even if spice-gtk was built with the gtk3 use flag so the check so
>=net-misc/spice-gtk-0.6[python,sasl?,-gtk3] should become >=net-misc/spice-gtk-0.6[python,sasl?]
It should build and run fine, until you connect to a SPICE based VM and try to use the remote video over SPICE. Unless of course you're using spice-gtk-0.14-r1 or newer and then it does in fact work (which was added today).
Should be fixed in the tree.