qemu: blockjob: Add modern block job event handler

qemu: blockjob: Add modern block job event handler

Add the infrastructure to handle block job events in the -blockdev era.

Some complexity is required as qemu does not bother to notify whether
the job was concluded successfully or failed. Thus it's necessary to
re-query the monitor.

To minimize the possibility of stuck jobs save the state into the XML
prior to handling everything so that the reconnect code can potentially
continue with the cleanup.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>