The XML parser sets a default <mode> if none is explicitly passed in.
This is then used at pool/vol creation time, and unconditionally reported
in the XML.

The problem with this approach is that it's impossible for other code
to determine if the user explicitly requested a storage mode. There
are some cases where we want to make this distinction, but we currently
can't.

Handle <mode> parsing like we handle <owner>/<group>: if no value is
passed in, set it to -1, and adjust the internal consumers to handle
it.