There's a RHEL developer ISO available to download at this location: https://developers.redhat.com/products/rhel/get-started/ When installing it in GNOME Boxes, express install isn't offered as an option. It would be really great if it could be, as this would dramatically cut down the amount of work in installing it as a VM. I was advised by developers on the GNOME Boxes IRC channel that this was a libosinfo issue.
As found out by Christophe and mentioned in the libosinfo mailing list, the strings in the ISO header are 32 bytes long and padded with spaces (0x20), but nothing in on on_pvd_read()/on_svd_read() is removing these padding spaces. I'm cooking up a patch for this that will be sent to the mailing list soon.
Here's the libosinfo commit fixing the issue: https://gitlab.com/libosinfo/libosinfo/commit/a75dc46e27cedb4562719c00661376c153fb83d2 And here's the osinfo-db commit working the issue around till we have a libosinfo release: https://gitlab.com/libosinfo/osinfo-db/commit/3ca9c9e2bfc7e3114858bcb33cdddc87230b7816
osinfo-db-20170107-1.fc25 has been submitted as an update to Fedora 25. https://bodhi.fedoraproject.org/updates/FEDORA-2017-ed34b2ad09
osinfo-db-20170107-1.fc25 has been pushed to the Fedora 25 testing repository. If problems still persist, please make note of it in this bug report. See https://fedoraproject.org/wiki/QA:Updates_Testing for instructions on how to install test updates. You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-ed34b2ad09
osinfo-db-20170107-1.fc25 has been pushed to the Fedora 25 stable repository. If problems still persist, please make note of it in this bug report.