[PATCH v2] qmgr: Call the notification callback when jobs complete.

[Erich E. Hoover]

v2: Fix the type of the job provided to the callback
(access through *_iface, typecast from IBackgroundCopyJob3 to
IBackgroundCopyJob)

The MSVC++ 2015 build tools installer expects the job notification
callback to get called when the job is completed (even though it
appears to also poll the job).  With this patch the callback gets
called appropriately and the installer is able to proceed past the
"Creating system restore point..." red herring and complete
successfully.

Best,
Erich