virsh: Try to keep printed XML pretty with change-media
    
    When adding a new media with change-media and --print-xml, let's try
    making it more readable and nice.
    
    Before:
    <disk type="file" device="cdrom">
    ...
          <target dev="hdb" bus="ide"/>
          <address type="drive" controller="0" bus="0" target="0" unit="1"/>
        <source file="/tmp/a.iso"/></disk>
    
    After:
    <disk type="file" device="cdrom">
    ...
          <source file="/tmp/a.iso"/>
          <target dev="hdb" bus="ide"/>
          <address type="drive" controller="0" bus="0" target="0" unit="1"/>
        </disk>
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1219719
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>