[XEN][POWERPC] Allocate Xen memory area based on the amount of memory

[XEN][POWERPC] Allocate Xen memory area based on the amount of memory
This will make sure that there is enough memory for large HTABs, as well as:
 - enables "xenheap_megabytes=" cmdline option
 - consistently reports on memory system
 - reduces noise in memory.c
Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
--HG--
extra : transplant_source : %C7%FA%7CA%B9%E6%0C%DD%DD%F9%0F%ED%DE%A8%C1_%01_Ck