Fix the memory reservation calculations. Introduce a new architecture-specific

Fix the memory reservation calculations.  Introduce a new architecture-specific
call to determine the initial reservation separately from the amount of
memory to be freed up by ballooning -- when using QEMU, we lose 8 MiB to video
RAM, which must be accounted for when ballooning, but _not_ accounted for
within the domain's memory allocation.

Tidy this code up using the new architecture-specific interface arrangement.

Signed-off-by: Ewan Mellor <ewan@xensource.com>