hvmloader: Add a simple "scratch allocator"

hvmloader: Add a simple "scratch allocator"

Returns memory which is passed to the subsequent BIOS but can be
reused once the contents is consumed. An example of this would be a
BIOS table which the BIOS consumes by copying rather than simply
referencing.

Users which need a temporary scratch buffer for internal use
scratch_start which follows these allocations.

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