Introduce public API for cancelling async domain jobs

The new virDomainAbortJob() method provides a way for a second
thread to abort an ongoing job run by another thread. This
extends to any API with  which the virDomainGetJobInfo() API
is intended to work. Cancellation is not guarenteed, rather best
effort on part of the hypervisor and not required to be implmented.

* include/libvirt/libvirt.h.in: Define virDomainAbortJob()

include/libvirt/libvirt.h.in

@@ -1834,6 +1834,7 @@ struct _virDomainJobInfo {
 
 int virDomainGetJobInfo(virDomainPtr dom,
                         virDomainJobInfoPtr info);
+int virDomainAbortJob(virDomainPtr dom);
 
 
 #ifdef __cplusplus