core: adding RetrieveImageDataCommand

This patch adds RetrieveImageDataCommand and changes
AttachStorageDomainToPoolCommand to use it instead of executing the
vds command directly. This is done so that the needed CDA checks/locks
will be taken. Furthermore, this patch takes care of polling the created
upload task on vdsm side.

Change-Id: I4ed10f1d3fc74e1deb6c69fc826f02abfd5d679e
Bug-Url: https://bugzilla.redhat.com/1136840
Signed-off-by: Liron Aravot <laravot@redhat.com>