Description of problem:
blockpull output is diffrent for diffrent size of qed domain img without backing file. 
For 50M qed domain img without backing file, the following error will prompt: 
# virsh blockpull vr-rhel6-x86_64-kvm /var/lib/libvirt/images/qed 1
error: Requested operation is not valid: No active operation on device: drive-ide0-0-0
For 1G qed domain img without backing file, blockpull can be succeed to executed:
# virsh blockpull vr-rhel6-x86_64-kvm /var/lib/libvirt/images/qed 1

Version-Release number of selected component (if applicable):
libvirt-0.9.8-1.el6.x86_64
qemu-kvm-0.12.1.2-2.213.el6.x86_64
kernel-2.6.32-220.el6.x86_64

How reproducible:
everytimes

Steps to Reproduce:
For 50M qed domain img without backing file
1. create a 50M qed img 
# qemu-img create -f qed /var/lib/libvirt/images/qed  50M
Formatting '/var/lib/libvirt/images/qed', fmt=qed size=52428800 cluster_size=0 table_size=0
2. start guest using the qed img
...
<disk type='file' device='disk'>
  <driver name='qemu' type='qed'/>
  <source file='/var/lib/libvirt/images/qed'/>
  <target dev='hda' bus='ide'/>
</disk>
...
# virsh start vr-rhel6-x86_64-kvm
Domain vr-rhel6-x86_64-kvm started
3. start to block pull
# virsh blockpull vr-rhel6-x86_64-kvm /var/lib/libvirt/images/qed 1
error: Requested operation is not valid: No active operation on device: drive-ide0-0-0

For 1G qed domain img without backing file:
1. create a 1G qed img 
# qemu-img create -f qed /var/lib/libvirt/images/qed  1G
Formatting '/var/lib/libvirt/images/qed', fmt=qed size=1073741824 cluster_size=0 table_size=0
2. start guest using the qed img
...
<disk type='file' device='disk'>
  <driver name='qemu' type='qed'/>
  <source file='/var/lib/libvirt/images/qed'/>
  <target dev='hda' bus='ide'/>
</disk>
...
# virsh start rhel6
Domain rhel6 started
3. start to block pull
# virsh blockpull rhel6 /var/lib/libvirt/images/qed 1
  
# virsh blockjob rhel6 /var/lib/libvirt/images/qed --info
Block Pull: [ 30 %]    Bandwidth limit: 1 MB/s

Actual results:
blockpull output is diffrent for diffrent size of qed domain img without backing file. 

Expected results:
If blockpull doesn't support for qed domain img without backing file, the error should raised regardless of the size of img.

Additional info: