qemu: Fix crash in offline migration
    
    When migrating a shutoff domain (i.e., offline migration), we have no
    statistics to report and thus jobInfo will be NULL in
    qemuMigrationFinish.
    
    Broken by me in v3.10.0-183-ge8784e7868.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1536351
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>