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

These two patches add support for using a Unix domain socket to directly access imageio in the case where imageio is running on the conversion host (usually that means virt-v2v is running on the RHV node and something else -- eg. CFME scripts -- arranges that the RHV node is the same one running imageio). Conversions in the normal case are not affected - they happen over TCP as usual. This was extremely hard to test, but I did eventually manage to test it both ways. The log from the Unix domain socket case is here: https://paste.fedoraproject.org/paste/RDhnpSaoUGI-jccIKy2e-g You can tell that the optimization was used because you will see this in the debug output: disk.id = 'c5bb72bf-b9ab-4797-9ecf-fcbf79679742' host.id = '87daa64d-b27e-4336-a156-973544be708b' transfer.id = '2207ddda-00fd-4caf-b02c-e025e8173da9' imageio features: flush=True trim=False zero=True unix_socket='\x00/org/ovirt/imageio' optimizing connection using unix socket '\x00/org/ovirt/imageio' (Note are the references to the unix socket.) This requires the absolute latest versions of imageio (1.4.0) and ovirt-engine (4.2.4-5) in order to get the optimization. However it should work (without optimization) with older versions. Thanks Nir Soffer and Daniel Erez in particular for help with this. Rich.