With the introduction of the `by_path` introspection
attribute at the IPA [1], it is now possible to leverage
the stable block device names for root device hinting.
This patch adds the `by_path` attribute to the collection
of valid root device hints.
1. https://review.openstack.org/#/c/498489/
Change-Id: I6f1c6b5ec5face7180aeb2d1aeebe50a6eb33f24