qemuBlockJobProcessEventCompletedPull: Add backingStore terminators if base is NULL

qemuBlockJobProcessEventCompletedPull: Add backingStore terminators if base is NULL

When doing a blockpull with NULL base the full contents of the disk are
pulled into the topmost image which then becomes fully self-contained.

qemuBlockJobProcessEventCompletedPull doesn't install the backing chain
terminators though, although it's guaranteed that there will be no
backing chain behind disk->src.

Add the terminators for completness and for disabling backing chain
detection on further boots.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>