Currently qbs-1.7.0 is the highest version available in portage, while dev-util/qt-creator-4.3.0_beta1 needs higher version for some purposes (such as "Plain C Project / QBS". So, qbs-1.7.1 is needed for recent qt-creator. Simple copying 1.7.0 ebuild to local overlay with new version works OK
Bumped to 1.8.0 https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=95bf1f6e1221c7a74f63b8406602cb9d1be0105d