iotests: Test node/bitmap aliases during migration

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200820150725.68687-4-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
[eblake: fold in python cleanups recommended by Vladimir]
Signed-off-by: Eric Blake <eblake@redhat.com>

tests/qemu-iotests/300.out

+.....................................
+----------------------------------------------------------------------
+Ran 37 tests
+
+OK

tests/qemu-iotests/group

@@ -307,6 +307,7 @@
 296 rw
 297 meta
 299 auto quick
+300 migration
 301 backing quick
 302 quick
 303 rw quick