OS install failed in the VM created using an ISO image

When we install OS in a VM created by an ISO image, it was failed.

Steps to reproduce:
1. create an ISO image, glance image-create --name ubuntu.iso --disk-format iso --container-format bare --file ubuntu-14.04.2-server-amd64.iso --progress
2. create a vm using this ISO image
3. install OS in this vm.

We think the nova generates wrong libvirt config, below are two examples:
1. For a vm with local storage, it can't detect disk when install OS. The partial libvirt.xml is as follow:

    <disk type='file' device='cdrom'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/nova/instances/6ee52fd1-ad97-41b0-8980-3d4dab3b11ba/disk'/>
      <backingStore type='file' index='1'>
        <format type='raw'/>
        <source file='/var/lib/nova/instances/_base/1d20447e6c2e83c4e6a98167b834fda2c5cb667d'/>
        <backingStore/>
      </backingStore>
      <target dev='hda' bus='ide'/>
      <readonly/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

2. For a volume vm, it can't detect CD-ROM when install OS. And the partial libvirt.xml is as follow:

    <disk type='network' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <auth username='compute'>
        <secret type='ceph' uuid='a5d0dd94-57c4-ae55-ffe0-7e3732a24455'/>
      </auth>
      <source protocol='rbd' name='volumes/volume-89608745-82c0-4717-bfe5-064c1b5785cd'>
        <host name='10.133.1.3' port='6789'/>
        <host name='10.133.1.9' port='6789'/>
        <host name='10.133.1.10' port='6789'/>
      </source>
      <backingStore/>
      <target dev='vda' bus='virtio'/>
      <serial>89608745-82c0-4717-bfe5-064c1b5785cd</serial>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>