libxl/x86: Build e820 map earlier for HVM/PVH guests

libxl/x86: Build e820 map earlier for HVM/PVH guests

Since hvm_start_info has now been expanded to include memory map (i.e.
e820) we need to know size of this map by the time we create
dom->start_info_seg in alloc_magic_pages_hvm().

To do so we have to call libxl__arch_domain_construct_memmap() earlier,
before xc_dom_build_image(). And since libxl__arch_domain_construct_memmap()
is only used by for x86 we can make this call from x86's
libxl__arch_domain_finalise_hw_description(), at the same time removing
its NOP definition from ARM code and renaming and making it static in
libxl_x86.c

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>