A backup blockjob needs to be able to notify the parent backup job as
well as track all data to be able to clean up the bitmap and blockdev
used for the backup.

Add the data structure, job allocation function and status XML formatter
and parser.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>