Add SystemType to Systems/hypervisor

Add SystemType to hypervisor page. Set Systemtype to 'OS'
Redfish Schema definition of 'OS' [1]
"OS": "A SystemType of OS typically represents an OS or hypervisor
view of the system.",

[1] https://redfish.dmtf.org/schemas/v1/ComputerSystem.v1_15_0.json

Testing:
1) Passed Redfish validator
2) Curl testing

curl -k -H "X-Auth-Token: $token" https://$bmc/redfish/v1/Systems/hypervisor
{
  "@odata.id": "/redfish/v1/Systems/hypervisor",
  "@odata.type": "#ComputerSystem.v1_6_0.ComputerSystem",
  ...
  "SystemType": "OS"
}

Signed-off-by: Ali Ahmed <ama213000@gmail.com>
Change-Id: If2a68e592303f1d862864fc06b6d33c146cb3450
diff --git a/redfish-core/lib/hypervisor_system.hpp b/redfish-core/lib/hypervisor_system.hpp
index 8aa9a44..107a5fa 100644
--- a/redfish-core/lib/hypervisor_system.hpp
+++ b/redfish-core/lib/hypervisor_system.hpp
@@ -748,6 +748,7 @@
                         asyncResp->res.jsonValue["Description"] = "Hypervisor";
                         asyncResp->res.jsonValue["Name"] = "Hypervisor";
                         asyncResp->res.jsonValue["Id"] = "hypervisor";
+                        asyncResp->res.jsonValue["SystemType"] = "OS";
                         asyncResp->res.jsonValue["Links"]["ManagedBy"] = {
                             {{"@odata.id", "/redfish/v1/Managers/bmc"}}};
                         asyncResp->res.jsonValue["EthernetInterfaces"] = {