[XEN] The shadow FAST_FAULT_PATH optimisation assumes that pages never

[XEN] The shadow FAST_FAULT_PATH optimisation assumes that pages never
transition between mmio and RAM-backed.  This isn't true after a
an add_to_physmap memory op.  Fix this by just blowing the shadow tables
after every such operation; they're rare enough that the performance
hit is not a concern.

Signed-off-by: Steven Smith <sos22@cam.ac.uk>
Acked-by: Tim Deegan <Tim.Deegan@xensource.com>