[powerpc] domain building fixes for linux kexec model

[powerpc] domain building fixes for linux kexec model
The following updates are included:
  - No stack allocation is necessary
  - Some buggy kernels require r13 to be zeroed
  - the DTB must be loaded from a fixed address, we are using
    "/root/DomU.dtb" until we have the tools build the DTB on their
    own.
  - Though we give the PFN of the store and console pages to the new
    domain we must make sure the MFN is given to the tools.

Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>