In order to add a string to qemuDomainJobInfo we must ensure that it's
freed and copied properly. Add helpers to copy and free the structure
and adjust the code to use them properly for the new semantics.

Additionally also allocation is changed to g_new0 as it includes the
type and thus it's very easy to grep for all the allocations of a given
type.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>