datapath-windows: Support for IRP cancelling mechanism

datapath-windows: Support for IRP cancelling mechanism

Under certain circumstances, we might need to cancel a pending IRP
that has been submitted and not yet responded. This might occur when
the request takes too long to complete or when the process which
initiated the request terminated, leaving the request outstanding.

This patch provides this missing piece by adding support for IRP
cancelling mechanism.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/95
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>