fix an infinite loop when attaching an ISO to non-WebVirtMgr domain

Infinite loop happens when domain XML is created outside of WebVirtMgr. WVM expects CD-ROM entry after hard drives.

Failing:
```xml
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/iso/archlinux-2014.09.03-dual.iso'/>
      <target dev='hda' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='1' target='0' unit='1'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/ssd/5be1d57b-d5b1-4278-a807-d8dfefe3598a_vda.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>
```

Working:
```xml
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/hdd/sdfsdfsdf.img'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/iso/FreeBSD-10.0-RELEASE-amd64-dvd1.iso'/>
      <target dev='hda' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='1' target='0' unit='1'/>
    </disk>
```

This fixes it. Added a simple Python equivalent of [non-first level `break`](http://stackoverflow.com/a/189685/504845).