Pull up following revision(s) (requested by manu in ticket #1380):

sys/arch/x86/acpi/acpi_machdep.c: revision 1.27,1.28 (patch)

Attempt to obtain ACPI RSDP from the hypervisor for Xen PV
There are three possible way of obtaining the ACPI RSDP

- From Extended BIOS Data Area (EBDA) when kernel or Xen was booted from
  BIOS bootstrap
- From EFI SystemTable when kernel is booted from EFI bootstrap
- When Xen is booted from EFI bootstrap, EBDA is not mapped, and EFI
  SystemTable is not passed to the kernel. The only way to go is to
  obtain ACPI RSDP trhough an hypercall.

Note: EFI bootstrap support for booting Xen has not yet been committed.

Cast physical addresses via uintptr_t to ACPI_PHYSICAL_ADDRESS to deal
with all size variants of the types used here in different builds.

(martin)