qemu-iotests/199: add source-killed case to bitmaps postcopy

Previous patches fixes behavior of bitmaps migration, so that errors
are handled by just removing unfinished bitmaps, and not fail or try to
recover postcopy migration. Add corresponding test.

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: <20200727194236.19551-22-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>