Running into this issue and decided to post in case it helps someone else.
Using version 5.8.3, VM won’t boot off a custom ISO even though ISO is bootable.
CentOS-7-x86_64-Minimal-1908-Kickstart-Context-V5.iso: # ISO 9660 CD-ROM filesystem data 'CentOS 7 x86_64' (bootable)
Shutting down the VM and restarting doesn’t help. What works is this:
1) Power Off (Hard) 2) onevm updateconf <ID> 4) Save it 5) Try to boot off the ISO again. 6) Should now get an OS boot screen.