qemu: blockjob: Synchronously update backing chain in XML on ABORT/PIVOT
    
    When the synchronous pivot option is selected, libvirt would not update
    the backing chain until the job was exitted. Some applications then
    received invalid data as their job serialized first.
    
    This patch removes polling to wait for the ABORT/PIVOT job completion
    and replaces it with a condition. If a synchronous operation is
    requested the update of the XML is executed in the job of the caller of
    the synchronous request. Otherwise the monitor event callback uses a
    separate worker to update the backing chain with a new job.
    
    This is a regression since 1a92c719101e5bfa6fe2b78006ad04c7f075ea28
    
    When the ABORT job is finished synchronously you get the following call
    stack:
     #0  qemuBlockJobEventProcess
     #1  qemuDomainBlockJobImpl
     #2  qemuDomainBlockJobAbort
     #3  virDomainBlockJobAbort
    
    While previously or while using the _ASYNC flag you'd get:
     #0  qemuBlockJobEventProcess
     #1  processBlockJobEvent
     #2  qemuProcessEventHandler
     #3  virThreadPoolWorker