We ought to consider backporting the following patches for offline disk-only snapshot creation:

30f1bcc     snapshot: qemu: Fix detection of external snapshots when deleting
9b5a514     snapshot: qemu: Add support for external inactive snapshots

In POST:
http://post-office.corp.redhat.com/archives/rhvirt-patches/2012-November/msg00191.html

verify this bug with :
libvirt-0.10.2-10.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.337.el6.x86_64


# virsh snapshot-create-as bug inactive-external --disk-only
Domain snapshot inactive-external created

# virsh list 
 Id    Name                           State
----------------------------------------------------

# virsh snapshot-list bug --external --inactive
 Name                 Creation Time             State
------------------------------------------------------------
 inactive-external    2012-11-27 17:39:50 +0800 shutoff

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2013-0276.html