Add function for storing the reason trail of an instance

We are interested in storing the reason trail for operations that changed
the state of instances. This commit adds a function to do this.

Also, the GetInstReasonFilename() function name is prepended with an
underscore because it is only used in this file.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>