grub-bhyve: support overriding just --root flag

[Christian Schwarz]

Hi Alan,

> Does libvirt support using the bhyve UEFI-CSM firmware instead? That
> would let the VM boot using the native grub installed inside the VM, and
> avoid this issue entirely. It also makes starting a bhyve a single
> command instead of 2.

Thanks for the tip, I just converted the disk to GPT and now use UEFI directly.

The only problem I encountered with the UEFI firmware is that it will always
prefer the virtualized cdrom over hdd or, more generally, one cannot define a
boot order.

This gist contains a working minimal UEFI-only libvirt domain:

https://gist.github.com/problame/79a94ae05f5b17e11c3b5bc2fe5910c8

If you have any idea how to set the boot order via bhyve command line flags
I would be "happy" to patch libvirt to support this feature.

Otherwise, I hope this helps anyone reading this in the future,

  Christian