Indent correctly the instance-libvirt.xml

[obino]

Before the output would have lines like:

<devices><rng model="virtio"><backend model="random">/dev/urandom</backend></rng><disk type="file" device="floppy"><driver cache="none"/><source file="/home/eucalyptus/instances/work/xxxxxx/yyyy/floppy"/><target dev="fda"/></disk><disk type="block">

After:

    <rng model="virtio">
      <backend model="random">/dev/urandom</backend>
    </rng>
    <disk type="file" device="floppy">
      <driver cache="none"/>
      <source file="/home/eucalyptus/instances/work/AIDAAPBBFTSG4COXVGU7Q/i-35cc9e3f2db173cae/floppy"/>
      <target dev="fda"/>
    </disk>```