datapath-windows: event read should not fail when no events

The semantics are read operation are generally to return 0 bytes and
STATUS_SUCCESS when there are no events.

Also, added a fix to assign the PID to the synthetic OVS_MESSAGE formed
for the command validation.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>