sheepdog: add support for connecting to unix domain socket

This patch adds support for a unix domain socket for a connection
between qemu and local sheepdog server.  You can use the unix domain
socket with the following syntax:

 $ qemu sheepdog+unix:///<vdiname>?socket=<socket path>[#snapid]

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/qemu-options.hx b/qemu-options.hx
index e8fb78c..f598d7a 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -2100,7 +2100,7 @@
 
 Syntax for specifying a sheepdog device
 @example
-sheepdog[+tcp]://[host:port]/vdiname[#snapid|#tag]
+sheepdog[+tcp|+unix]://[host:port]/vdiname[?socket=path][#snapid|#tag]
 @end example
 
 Example