live: hw_inventory: fix empty pci storage size bug

When a client's hardware presents an empty pci storage child there is an
invalid call to _bytes_to_human a string is supplied as a default value
if the storage child does not present a 'size' attribute.

Fix this by checking if 'size' is present in the JSON output from lshw.
If size is present then map the bytes to a human readable string using
_bytes_to_human, if no size is present then use 'Empty slot' to indicate
that the memory bank is not being used.

src/utils/hw_inventory.py

@@ -151,7 +151,7 @@ def _process_core_pci_network(inventory, obj):
 def _process_core_pci_storage_child(inventory, obj):
     obj_id = obj.get('id', '')
     if obj_id.startswith('disk') or obj_id.startswith('nvme'):
-        size = _bytes_to_human(obj.get('size', 'Unknown size'))
+        size = _bytes_to_human(obj['size']) if 'size' in obj else 'Unknown size'
         name = ' '.join([obj.get('description', ''), obj.get('product', 'Unknown product'), size])
         elem = HardwareElement(HardwareType.DISK, name)
         inventory.add_element(elem)