Noticed that the expected "not supported" error is dropped when
invoking 'virsh snapshot-list dom' on a Xen installation running
the libxl driver
virsh snapshot-list test
error: Invalid snapshot: virDomainSnapshotFree
The error is overwritten by a call to virDomainSnapshotFree
in cleanup code within cmdSnapshotList. Prevent overwritting
the real error by not calling virDomainSnapshotFree with a NULL
virDomainSnapshotPtr.
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