libxl: drop 8M slack for PV guests.

libxl: drop 8M slack for PV guests.

This serves no purpose. It relates to the old 8M to "account for backend
allocations" which we used to add. This leaves a bit of unpopulated space in
the Pseudo-physical address space which can be used by backends when mapping
foreign memory. However 8M is not representative of that any more and modern
kernels do not operate in this way anyway.

I suspect an argument could be made for removing this from the libxl API
altogether but instead lets just set the overhead to 0.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>