When the guest XML uses the old boot syntax ... <os> ... <boot dev='hd'/> </os> ... virt-manager incorrectly marks all disks as bootable in the "Boot Options" pane in guest details window. That's not true, only the fist disk is bootable in this case.
Upstream patch posted: https://www.redhat.com/archives/virt-tools-list/2017-September/msg00012.html
Upstream commit: commit f14b1f88a092b9eea9c74626dcb0f7b258bba5a3 Author: Pavel Hrdina <phrdina> Date: Tue Sep 5 15:48:28 2017 +0200 domain: only the first device from each group is bootable