fixed xpath duplicate origin

Using a custom origin in the sensor path wasn't split up properly causing the origin to be appended to the first element of the path which led to invalid sensor paths

Author: marcsello

src/cisco_gnmi/xr.py

@@ -338,5 +338,6 @@ def parse_xpath_to_gnmi_path(self, xpath, origin=None):
                 origin = None
             else:
                 # module name
-                origin = xpath.split(":")[0]
+                origin, xpath = xpath.split(":")
+
         return super(XRClient, self).parse_xpath_to_gnmi_path(xpath, origin)