core: fix storage pool ID association with tasks

The following patch fixes the association of storage
pool ID to its task after the task is created at VDSM.
Without this fix, the storage_pool_id column at async_tasks
table will have entries of EMPTY guids.

Bare in mind that when a place holder is created, the
async task creation info does not have the storage pool ID information.
IRS broker commands (i.e - CopyImage) create an async task creation info
object with storage pool ID, once the task is created at SPM and the task ID
is returned to engine.

Change-Id: I1b8f06e7c3b16504f76f6188f3d99fa5e80eedd4
Bug-Url: https://bugzilla.redhat.com/990524
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>