qemu-iotests/199: add early shutdown case to bitmaps postcopy

Previous patches fixed two crashes which may occur on shutdown prior to
bitmaps postcopy finished. Check that it works now.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Tested-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200727194236.19551-21-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>