With the introduction of the `bootloader` option, grub2 legacy
installation setting changed. Before, grub2 legacy installation
was dependent on the partition scheme, i.e. only when dos/mbr
layout was used grub2 got installed. After the change the default
is to install it unless `bootloader.type" is explicitly set, even
if the partition layout is GPT. But a legacy grub2 installation
on GPT requires a BIOS boot partition, so the new default is not
right for the case of pure (non-hyrid) UEFI images.
Therefore revert to the old behavior of only defaulting to grub2
legacy if the option is not explicitly set *and* the partition
layout is "dos"/"mbr".
Adapt the f30-qcow2-gpt sample, which is non-uefi grub2 legacy
but with GPT and a bios boot partition, to explicitly request
the grub2 bootloader.
