qemu: block: Introduce helper for deciding when a 'format' layer is needed

qemu: block: Introduce helper for deciding when a 'format' layer is needed

The 'format' layer is not required in certain cases. As the logic for
this will be a bit more involved create a helper function to do the
decision.

For now we'll keep to always format the 'format' -blockdev layer.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>