From the ebuild: src_prepare() { epatch "${FILESDIR}"/${P}-optional_avahi.patch distutils-r1_src_prepare } However, there's no virt-manager-9999-optional_avahi.patch file. Moreover, the patch does only log the avahi server version. So I suggest dropping the epatch line completely.
13 Apr 2015; Matthias Maier <tamiko@gentoo.org> virt-manager-9999.ebuild: fix src_prepare in live build; bug #546400