Description of problem:
libvirt do not report an error when failing to set <boot order> to cdrom device.

Version-Release number of selected component (if applicable):
libvirt-0.10.2-23.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.398.el6.x86_64
kernel-2.6.32-415.el6.x86_64

This issue can be reproduced on rhel6.4.z.
libvirt-0.10.2-18.el6_4.11.x86_64
qemu-kvm-0.12.1.2-2.355.el6.x86_64
kernel-2.6.32-358.el6.x86_64

How reproducible:
100%

Steps:
1. Define a guest with cdrom device.
[root@test666 ~]# virsh dumpxml r6 | grep "cd" -A7
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/ubuntu-12.04-server-amd64.iso'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
      <alias name='ide0-1-0'/>
      <address type='drive' controller='0' bus='1' target='0' unit='0'/>
    </disk>
2. Make a new cdrom xml as below, just add one line <boot order>
[root@test666 ~]# cat cdrom.xml
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<source file='/var/lib/libvirt/images/ubuntu-12.04-server-amd64.iso'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<boot order='2'/>
<address type='drive' controller='0' bus='2' target='0' unit='0'/>
</disk>

3.Update cdrom device with update-device command, and check the result of update.
[root@test666 ~]# virsh update-device r6 cdrom.xml
Device updated successfully

[root@test666 ~]# virsh dumpxml r6 | grep "cd" -A7
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/ubuntu-12.04-server-amd64.iso'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
      <alias name='ide0-1-0'/>
      <address type='drive' controller='0' bus='1' target='0' unit='0'/>
    </disk>
[root@test666 ~]#

Actual results:
As step 3, No boot order was added to domain xml, and that libvirt printed device updated successfully.

Expected results:
libvirt should print the error like the interface device or others if fail.

[root@test666 ~]#
[root@test666 ~]# virsh dumpxml r6 | grep "interface" -A7
    <interface type='network'>
      <mac address='52:54:00:da:0f:ac'/>
      <source network='default'/>
      <target dev='vnet0'/>
      <model type='virtio'/>
      <link state='down'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
...
[root@test666 ~]# virsh update-device r6 network.xml
error: Failed to update device from network.xml
error: this function is not supported by the connection driver: cannot modify network device boot index setting

For floppy device:

[root@test666 ~]# cat floppy.xml 
<disk type='file' device='floppy'>
<driver name='qemu' type='raw' cache='none'/>
<source file='/var/lib/libvirt/images/fd2.img'/>
<target dev='fda' bus='fdc'/>
<boot order='3'/>
<address type='drive' controller='0' bus='4' target='0' unit='0'/>
</disk>
[root@test666 ~]# virsh update-device r6 floppy.xml 
Device updated successfully

[root@test666 ~]# virsh dumpxml r6 | grep floppy -A6
    <disk type='file' device='floppy'>
      <driver name='qemu' type='raw' cache='none'/>
      <source file='/var/lib/libvirt/images/fd2.img'/>
      <target dev='fda' bus='fdc'/>
      <alias name='fdc0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
[root@test666 ~]#