Various QEMU SnapshotModule fixes

[wfdewith]

Description

• The snapshot module needs to clear the dirty pages when a new snapshot is made. When you take two snapshots, A and B, without resetting in between, and then you reset, the module will try to restore pages that have been written between snapshot A and B. These pages aren't saved because the saved pages are actually cleared when snapshot B is made, so it will hit the Cannot restore a dirty but unsaved page panic in the reset function.
• We shouldn't use println for debug logging. In addition, the MapInfo type has a nice Display implementation that is much more readable than the Debug implementation.
• The reset function uses the AllowList/DenyList to determine which pages to zero out on reset, which is incorrect. Instead, the snapshot function can skip saving the pages that should be skipped entirely, because that will cause the reset method to ignore them (pages.get_mut(page) will return None). However, the pages that should be zeroed (in a ZeroList) still need to be recorded during the snapshot stage, but their contents don't need to be read, so we just save the metadata.

Checklist

• I have run ./scripts/precommit.sh and addressed all comments