introduce virDomainJobWait for dump plus testing

This patch series introduces funcionality for waiting for a job from a different context, for now only for dump jobs, plus testing. This can be useful in situations where the original requester of the job crashes/restarts and then needs to continue waiting for that potentially ongoing job. To avoid waiting for the wrong job type, the caller must specify the required job type. For now I've only implemented this for dump jobs; if the logic is correct I can extended it for all other job types. The first patch introduces the core functionality. I see that there is a function specifically for waiting for a dump job: qemuDumpWaitForCompletion. Ideally we should only use qemuDomainObjWait, which would work for any job type, however I'm not sure it's correct to use it instead of qemuDumpWaitForCompletion because the latter checks dumpCompleted. It's not obvious to me what is the purpose of dumpCompleted, is it because qemuDomainObjWait can return because some other job, running in parallel with the dump job, has completed, so it's effectively a false alarm? The second patch is mainly for testing the wait functionality via virsh. The remaining patches extend the test hypervisor's functionality on dump testing.