core: generify "streamDownloadImage" related methods

This patch generifies the related "streamDownloadImage" methods in
image/imageSharing to be have more generic name as they could be used
with any passed fileObj.

Change-Id: I1c73374681b5a5fc9fd0cb81020138fb5c8bfe69
Signed-off-by: Liron Aravot <laravot@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26761
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>