Always use internal snapshots

Currently the code is pretty complex since we deal with both “memory-only” snapshots and internal snapshots. The only reason we use the former is that no qcow2 disk is present in some tests, and internal snapshots require that. However, we can use internal snapshots all the time if we just add a “dummy” qcow2 disk that is never used for anything except storing the snapshots.

Feature Branch: test/10974-use-only-internal-snapshots

Parent Task: #10237

Original created by @anonym on 10974 (Redmine)