core: imageSharing - export logic to functions

The "copy" logic within imageSharing.streamDownloadImage() can be
generified and exported out to be used by any operation. In this patch it's
being exported to the copyData function with the appopiate
generification.

Change-Id: I861b40cc62c3332b887b64c2525fc512cdc6a22a
Signed-off-by: Liron Aravot <laravot@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26759
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>