Virt-who connected to a HyperV fails with "NoneType object has no attribute __getitem__" in decodeWinUUID

Issue

Running virt-who against a HyperV hypervisor fails to collect the host to guest mapping, and ends up displaying a traceback like this:

2019-04-30 07:24:53,261 [virtwho.main ERROR] MainProcess(69762):Thread-7 @virt.py:run:397 - Thread 'virt-who-config-14' fails with exception:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/virtwho/virt/virt.py", line 388, in run
    self._run()
  File "/usr/lib/python2.7/site-packages/virtwho/virt/virt.py", line 343, in _run
    data_to_send = self._get_data()
--
    return HostGuestAssociationReport(self.config, self.getHostGuestMapping())
  File "/usr/lib/python2.7/site-packages/virtwho/virt/hyperv/hyperv.py", line 601, in getHostGuestMapping
    guests.append(virt.Guest(HyperV.decodeWinUUID(uuid), self.CONFIG_TYPE, state))
  File "/usr/lib/python2.7/site-packages/virtwho/virt/hyperv/hyperv.py", line 531, in decodeWinUUID
    if uuid[0] == "{":
TypeError: 'NoneType' object has no attribute '__getitem__'