Simplify -machine option queries with qemu_get_machine_opts()

The previous two commits fixed bugs in -machine option queries.  I
can't find fault with the remaining queries, but let's use
qemu_get_machine_opts() everywhere, for consistency, simplicity and
robustness.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1372943363-24081-7-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>