While working on a fix for #572304 I found that the xen-tools ebuild does call 'source' at two occasions, but it does not check whether or not statement was successful. This caused by experiments searching for a fix to derail because I had issues with my ebuild (the qemu-bridge-helper was not renamed). Turns out that was because I forget to copy the content of FILESDIR into my overlay, so the source "${FILESDIR}"/gentoo-patches.conf in line 214 failed unnoticed. I suggest using source "${FILESDIR}"/gentoo-patches.conf || die instead.
Created attachment 470432 [details, diff] xen-tools-ebuild-add-die.patch
fixed in tree, thanks for reporting this ;-) https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=911800c0fe06e2c32394d04a19af828257914bd6
