core: UploadImageVDSCommand - read task id from header

In the following patch [1] there's a behavior change, in case of a
succesfull execution the task id will be returned within a header
instead of the body. This patch just alignes the engine behavior to read
the task id from it.

[1] - http://gerrit.ovirt.org/#/c/27997/

Change-Id: I9d9d18fcefdc7862517d7d59aff18ba500e22c63
Signed-off-by: Liron Aravot <laravot@redhat.com>