vmstate: fix breakage by 7e72abc3

cirrus_post_load() will be executed twice when loading vm states and then the
wrong physical memory will be registered. This issue may lead to crash qemu.

Signed-off-by: TeLeMan <geleman@gmail.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>