exynos4: Declare each QEMUMachine as a separate variable

This will make the code follow the same pattern used for other machines,
and will make it easier to automatically convert the code to be
QOM-based.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>