boot-order-test: Better separate target-specific and generic parts

boot-order-test: Better separate target-specific and generic parts

The initial version did just PC.  I didn't bother to separate out
generic parts, because I don't like to abstract from a single case.

Now we have two cases, PC and PowerMac, and I'm about to add more.
Time to do it right.

To ease review, this commit changes the code in-place, and the next
commit reorders it for better readability.

Cc: Andreas Färber <afaerber@suse.de>
Cc: Alexander Graf <agraf@suse.de>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1372254743-15808-8-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>