iotests: Add (NBD-based) tests for inactive nodes

iotests: Add (NBD-based) tests for inactive nodes

This tests different types of operations on inactive block nodes
(including graph changes, block jobs and NBD exports) to make sure that
users manually activating and inactivating nodes doesn't break things.

Support for inactive nodes in other export types will have to come with
separate test cases because they have different dependencies like blkio
or root permissions and we don't want to disable this basic test when
they are not fulfilled.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250204211407.381505-17-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>