virsh: Try to keep printed XML pretty with change-media

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>