Unable to snapshot an instance with ephemeral RBD

When creating a snapshot from a VM instance, following error appears in Nova compute log:

  File "/usr/lib/python2.6/site-packages/nova/virt/libvirt/driver.py", line 1403, in snapshot
    snapshot_backend.snapshot_extract(out_path, image_format)
  File "/usr/lib/python2.6/site-packages/nova/virt/libvirt/imagebackend.py", line 658, in snapshot_extract
    images.convert_image(snap, target, out_format)
  File "/usr/lib/python2.6/site-packages/nova/virt/images.py", line 179, in convert_image
    utils.execute(*cmd, run_as_root=run_as_root)
  File "/usr/lib/python2.6/site-packages/nova/utils.py", line 177, in execute
    return processutils.execute(*cmd, **kwargs)
  File "/usr/lib/python2.6/site-packages/nova/openstack/common/processutils.py", line 178, in execute
    cmd=' '.join(cmd))
ProcessExecutionError: Unexpected error while running command.
Command: qemu-img convert -O raw rbd:compute/5fac0ad2-b39e-4006-90a8-b331ec38a57e_disk /var/lib/nova/instances/snapshots/tmpz0Nq0E/ab5eae70d5114c4e965de7163452934c
Exit code: 1
Stdout: ''
Stderr: "qemu-img: error connecting\nqemu-img: Could not open 'rbd:compute/5fac0ad2-b39e-4006-90a8-b331ec38a57e_disk': No such file or directory\nqemu-img: Could not open 'rbd:compute/5fac0ad2-b39e-4006-90a8-b331ec38a57e_disk'\n"

Apparently Nova doesn't have (or doesn't give qemu-img) enough permissions to access the VM image in RBD. When Rbd.snapshot_extract is patched to pass run_as_root to convert_image (which in turn runs qemu-img), snapshot is successfully created.