vl: Fix to create migration object before block backends again

Recent commit cda4aa9a moved block backend creation before machine
property evaluation.  This broke qemu-iotests 055.  Turns out we need
to create the migration object before block backends, so block
backends can add migration blockers.  Fix by calling
migration_object_init() earlier, right before configure_blockdev().

Fixes: cda4aa9a
Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>