Hi,
your package is still using the old %ruby_sitelib and %ruby_sitearch macros, which are now replaced by %ruby_vendorlibdir and %ruby_vendorarchdir. These are located in macros.ruby in ruby-devel package (therefore you don't need to define them). The old macros now point to /usr/local, which results in an unwanted placement of files of the ruby subpackage of libguestfs.