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 1a92c719

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