schema: Add virtual sensor schema

This schema allows entity manager to define virtual sensors that are
used by the virtual-sensor daemon from the phosphor-virtual-sensor
repository.

Currently only a modified median of sensor values is accepted by
phosphor-virtual-sensor.

Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I510dbcd843f703073a96ad33cfe748d0dcd21e64
diff --git a/schemas/global.json b/schemas/global.json
index 9385ffc..4151cc8 100644
--- a/schemas/global.json
+++ b/schemas/global.json
@@ -30,6 +30,9 @@
                     "$ref": "legacy.json#/definitions/Aggregate"
                 },
                 {
+                    "$ref": "VirtualSensor.json#/definitions/VirtualSensor"
+                },
+                {
                     "description": [
                         "Ensure the parent oneOf schema does not validate ",
                         "when an element does not define a type property."