Whole gentoo-x86 is using the virtuals now. Please check your overlays
to avoid dev-libs/libusb from creeping back in to the tree.
If you have packages in overlays using dev-libs/libusb please fix them
now to avoid them creeping into Portage.
There is a repoman bug open but still without a patch:
This is important now because of the dev-libs/libusb-compat stabilization:
Otherwise you'd be introducing conflicting dependencies for /stable/ users.
Futhermore some of the virtual/libusb dependencies in tree are missing
usage of correct SLOT, and should be fixed, that involves checking the
package sources (of course). If you want to help with this, please do.