Emerging xen-tools fails if iasl is not installed. Yet, the ebuild does not have a dependency on iasl. Reproducible: Always
I can also confirm the missing dependency with xen-tools-4.2.0-r1 on amd64
Now the ebuild with the missing dependency made it into the main tree. Please consider fixing this.
The copy of the -4.2.0 ebuild has a number of flaws. The fixes have taken place in the upgraded version now in portage, 11 Dec.; 4.2.0-r2