there is allready a global USE caps which does the same as the local USE filecaps for qemu. Please rename filecaps to caps to make it equal.
We use the fcaps eclass to handle all this. Talk to those authors. I personally prefer it to be broken out since there are definitely different meanings and USE=filecaps requires your filesystem to bet setup correctly.
this was discussed on the mailing list. please see the introductory thread for the explanation.