Hello,

Here is a patchset solving this issue and adding a new UEFI system
test. The boot issue was quite complex. Turns out it's related to how
the OVMF firmware saves persistently the UEFI variables.

Regarding the MSDOS/UEFI patch, it is almost identical to what Florian
tested. I chose not to force GPT on UEFI if the user disk is already
MBR formatted, mostly to keep the code readable.

Thanks,

Mathieu