gentoo-overlay/gnome-extra/gnome-boxes/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
	<maintainer type="project">
		<email>gnome@gentoo.org</email>
		<name>Gentoo GNOME Desktop</name>
	</maintainer>
	<use>
		<flag name="usbredir">Use <pkg>sys-apps/usbredir</pkg> to redirect USB
			devices to another machine over TCP</flag>
	</use>
</pkgmetadata>