xen: portability clean up and some minor clean up for xencomm.c

clean up of xencomm.c. is_phys_contiguous() is arch dependent
function that depends on how virtual memory are laid out.
So split out the function into arch specific code.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: "Luck, Tony" <tony.luck@intel.com>