Update list_instances to catch libvirtError.
Update list_instances to catch libvirtError.

Commit be590cfa4b0025587a956642dc859902d577f3f7 caught (and tested
for) the wrong exception. lookupByID throws a libvirtError.

Fixes LP Bug #1032741.

Change-Id: I6745660658df4d0f2bc4cd423e173200fc23cd8e

n0ano/gantt,n0ano/gantt