When importing a .dmg, ensure the resulting .qcow2 is bootable

As described in #6049, UTM's "Import" disk option does not "just work".

If you import a .dmg to create a VM OS to boot into, the resulting .qcow2 fails to boot with read errors, because the disk isn't using any partition format scheme.

UTM could make this "just work" by following [the recipe I described ](https://github.com/utmapp/UTM/discussions/6049#discussioncomment-9091929) and ensuring that the single-volume .dmg is converted into a disk image using a GUID (for Intel) or Apple (for PPC) partition scheme before importing it.