Introduce the handler for finalizing a block pull job which will allow
to use it with blockdev.

This patch also contains some additional machinery which is required to
store all the relevant job data in the status XML which will also be
reused with other block job types.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>