tools: hvmloader: split e820 support into its own code module.

tools: hvmloader: split e820 support into its own code module.

Pass the table address as a paramter to the build function and cause
it to return the number of entries. Pass both base and offset as
parameters to the dump function.

This adds a duplicated e820.h header to ROMBIOS. Since the e820 data
structure is well defined by existing BIOS implementations I think
this is OK and simplifies the cross talk between hvmloader and
ROMBIOS.

Reduces the cross talk between ROMBIOS and hvmloader.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>