python: Add binding for xs_fileno()

xs_fileno() returns a file descriptor which receives events when Xenstore
watches fire.   Exposing this in the Python bindings is a prerequisite
for writing event-driven clients in Python.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

tools/python/xen/lowlevel/xs/xs.c

@@ -453,6 +453,25 @@ static PyObject *xspy_watch(XsHandle *self, PyObject *args)
 }
 
 
+#define xspy_fileno_doc "\n"                              \
+	"Return the FD to poll for notifications when watches fire.\n"   \
+	"Returns: [int] file descriptor.\n"                \
+	"\n"
+
+static PyObject *xspy_fileno(XsHandle *self)
+{
+    struct xs_handle *xh = xshandle(self);
+    int fd;
+
+    if (!xh)
+        return NULL;
+
+    fd = xs_fileno(xh);
+
+    return PyLong_FromLong(fd);
+}
+
+
 #define xspy_read_watch_doc "\n"				\
 	"Read a watch notification.\n"				\
 	"\n"							\
@@ -887,6 +906,7 @@ static PyMethodDef xshandle_methods[] = {
     XSPY_METH(release_domain,    METH_VARARGS),
     XSPY_METH(close,             METH_NOARGS),
     XSPY_METH(get_domain_path,   METH_VARARGS),
+    XSPY_METH(fileno,            METH_NOARGS),
     { NULL /* Sentinel. */ },
 };