Installing Ubuntu 26.04 Server in an OpenBSD vmm(4) VM

I wanted to run Ubuntu 26.04 Server inside an OpenBSD vmm(4) guest. OpenBSD’s VM console is serial only, and the Ubuntu Server ISO does not boot correctly on a serial console out of the box. This post shows how to rebuild the Ubuntu ISO with the right kernel and GRUB options so it installs cleanly through vmctl console.