virdomainjob: add check for callbacks
virdomainjob: add check for callbacks

There may be a case that the callback structure will exist with
no callbacks (following patches). This patch adds check for
specific callbacks before using them.

Signed-off-by: Kristina Hanicova <e70d889c982bbe3b9ce2841af457ca911bff1baa@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

libvirt/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,olafhering/libvirt