migration: Move check_migratable() into qdev.c

The function is only used once, and nothing else in migration knows
about objects.  Create the function vmstate_device_is_migratable() in
savem.c that really do the bit that is related with migration.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>