I haven't tested but at first glance I would say that the target of the
mapped-device and the root filesystem device don't match: one is "root-part" and
the other is (file-system-label "root-fs").

According to the example in the manual ("(guix) Using the Configuration
System"), you should replace

	(file-system-label "root-fs")

with

  "root-part"

Also watch out with those `cons', you should use `cons*' or `list' if someday
you'd like to add more than one element.
-- 
Pierre Neidhardt