xen: Big changes to x86 start-of-day:

xen: Big changes to x86 start-of-day:

 1. x86/64 Xen now relocates itself to physical high memory. This is
    useful if we have devices that need very low memory, or if in
    future we want to grant a 1:1 mapping of low physical memory to a
    special 'native client domain'.

 2. We now only map low 16MB RAM statically. All other RAM is mapped
    dynamically within the constraints of the e820 map. It is
    recommended never to map MMIO regions, and this change means that
    Xen now obeys this constraint.

 3. The CPU bootup trampoline is now permanently installed at
    0x90000. This is necessary prereq for CPU hotplug.

 4. Start-of-day asm is generally cleaned up and diff between x86/32
    and x86/64 is reduced.

Signed-off-by: Keir Fraser <keir@xensource.com>