[Libguestfs] [PATCH v3] v2v: -o rhv-upload: Use Unix domain socket to access imageio (RHBZ#1588088).

v2 was here: https://www.redhat.com/archives/libguestfs/2018-June/msg00109.html v3: - Added/fixed all suggestions from Nir in previous review. Q: I wasn't sure if we still need the "UnsupportedError" class so I left it in. Q: Does the Unix socket always have the same name? What happens if there's more than one transfer happening? I tested this both ways, and it worked both ways. The output when the optimization is used looks like this: disk.id = '45d1a30c-82c0-4b00-8bfb-c4d628dbf72d' hw_id = '81F716C4-4D19-CB35-82F7-94C691189FEB' host.id = '87daa64d-b27e-4336-a156-973544be708b' transfer.id = '5ffea7fa-966c-42a4-a77a-3efd6dc45c34' imageio features: flush=True trim=False zero=True unix_socket='\x00/org/ovirt/imageio' optimizing connection using unix socket '\x00/org/ovirt/imageio' Rich.