Apr 19 17:27:36 virtualserver3 kernel: [14202.411373] type=1503 audit(1271723256.513:75): operation="open" pid=3147 parent=1 profile="libvirt-20030353-cdee-109a-1abb-dc4971d036c2" requested_mask="r::" denied_mask="r::" fsuid=0 ouid=0 name="/data/virtual/var/lib/libvirt/images/ubuntu910server64.qcow2"
Apr 19 17:27:36 virtualserver3 libvirtd: 17:27:36.718: error : qemudWaitForMonitor:1103 : internal error unable to start guest: qemu: could not open disk image /var/lib/libvirt/images/ubuntu910server64_delta.qcow2#012
These indicate that AppArmor is denying access to the backing store, thus, your delta image will not work.
You're going to need add a rule into /etc/apparmor.d/abstractions/libvirt-qemu so that the backing store can be read. Something like:
/data/virtual/var/lib/libvirt/images/ubuntu910server64.qcow2 r,
or
/data/virtual/var/lib/libvirt/images/* r,
.:. jeremy