Soft delete tmp migration volume

Previously the row for the temporary migration volume was deleted from
the database, which caused a foreign key violation for its admin
metadata. Regardless, the volume should have been soft deleted from the
database anyway, and this patch takes care of that.

Change-Id: I8e4942b0e21ec79c90aa95b18b5612b5b133fd62
Closes-Bug: #1219097
Merge "Soft delete tmp migration volume"