The option 'srcSpec' to virsh command find-storage-pool-sources
is optional for logical type of storage pool, but mandatory for
netfs and iscsi type.
When missing the option for netfs and iscsi, libvirt reports XML
parsing error due to null string option srcSpec.

before
error: Failed to find any netfs pool sources
error: (storage_source_specification):1: Document is empty
(null)

after:
error: pool type 'iscsi' requires option --srcSpec for source discovery