[POWERPC][XEN] Handle foreign page mappings correctly

[POWERPC][XEN] Handle foreign page mappings correctly

The following patch performs the following:
  - add free_rma() to go with allocate_rma()
  - allocate_rma() returns -EINVAL if an RMA is already allocated
  - PFN_TYPE_REMOTE is now PFN_TYPE_FOREIGN
  - better checking of page_info in pfn2mfn()
  - Stop using RPN and LPN and use PFN and MFN respectively
  - Increase/decrease foreign domain refcount when another domain
    maps/unmaps it

Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>