API: Introduce field for reporting temporary disk space usage of a domain job

API: Introduce field for reporting temporary disk space usage of a domain job

A pull mode backup job uses temporary disk images to hold the changed
parts of the disk while the client is copying the changes. Since usage
of the temporary space can be monitored but doesn't really fit any of
the existing stats fields introduce new fields for reporting this data.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>