Use the new schema conversion logic to remove the old schema table

Now that we use the magic schema detection, we don't need the schema/magic
table, so we can use the new schema update logic to remove it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Test that we correct fail updating an unknown database schema

Which in turn protects against clobbering an unrelated sqlite database.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Extend database update test

Put in some basic verification that the database update doesn't clobber
actual data in the original db.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>