datapath-windows:Remove user mode instance from pid hash on process termination

The instance should be removed from pid-hash when vswitchd.exe terminates so
the driver won't access it during packet miss (causing BSOD)

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