assemblers/qemu: fix backward compat for grub2

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.

samples/f30-qcow2-gpt.json

@@ -76,6 +76,7 @@
     {
       "name": "org.osbuild.qemu",
       "options": {
+        "bootloader": {"type": "grub2"},
         "format": "qcow2",
         "filename": "base.qcow2",
         "ptuuid": "29579f67-d390-43e7-bd96-dc8f5461171e",