Fix virsh snapshot-list error reporting

Fix virsh snapshot-list error reporting

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.