[XEN][POWERPC] Remove pfn2mfn() now that it is understood

[XEN][POWERPC] Remove pfn2mfn() now that it is understood
It is possible (thank you xm-test) to initiate a grant copy to a page
belonging to a dying domain, the page end up being owned by nobody
which is what the original test asserted.
Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
--HG--
extra : transplant_source : z%F3cm%A4%B9U%E5V%AF%19%AF%91%B063%B5e%CC%E2