cdrkit is dead for ages and with lots of known bugs, but this looks to hard require cdrkit, then this should be either fixed to allow cdrtools or other backends or probably be treecleaned
The reason for the cdrkit requirement is to pull in "genisoimage" and the isotools (isodump, isoinfo, etc.) If those are available from another package in Gentoo, then you just need to substitute those in the build dependencies, and I think no other change should be required.
[master cbb4bfa] app-emulation/libguestfs: This is also compatible with cdrtools (#591782), allow python 3.5, set slots for libpcre and ncurses, also set subslot for gobject-introspection (https://wiki.gentoo.org/wiki/Project:GNOME/Gnome_Team_Ebuild_Policies#dev-libs.2Fgobject-introspection:.3D_dependency).
