QEMU's command line visitor as well as the JSON interface take bytes by default for memory object sizes. Convert mebibytes to bytes so that we can later refactor the existing code for hotplug purposes.
Move the alias as the second formated argument and tweak the tests so that a future refactor that will change the order doesn't break tests.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>