Update XENBUS_EVTCHN interface with new Send method

Update XENBUS_EVTCHN interface with new Send method

The new version of the method has the same arguments and return of the
previous version but does not modify IRQL. It must therefore be called
with IRQL >= DISPATCH_LEVEL. Most callers already do this (usually
because they have a spin lock held) so the overhead of calling
KeRaiseIrql() and KeLowerIrql() can be saved. Also, it makes the method
safe to be called at > DISPATCH_LEVEL.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>