socket_connect() sets non-blocking on TCP or UNIX domain sockets if a
callback function is passed.  Do the same for file descriptor passing,
otherwise we could unexpectedly be using a blocking file descriptor.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>