daemon: Resolve Coverity FORWARD_NULL

daemon: Resolve Coverity FORWARD_NULL

Coverity complains that the comparison:

  if (nfds && nfds > ((int)!!sock_path + (int)!!sock_path_ro))

could mean 'sock_path' is NULL. Later in virNetSocketNewListenUNIX
there's a direct dereference of path in the error path:

  if (path[0] != '@')

A bit of sleuthing proves that upon entry to daemonSetupNetworking
there is no way for 'sock_path' to be NULL since daemonUnixSocketPaths
will set up 'sock_file' (although it may not set up 'sock_file_ro')
in all 3 paths.

Adjusted code to add ATTRIBUTE_NONNULL(3) on incoming path parameter and
then fixup the comparison of nfds to be a comparison against 2 or 1
depending on whether sock_path_ro is NULL or not.