Introduce page_to_bus() and use it in pci-dma-xen.c and swiotlb.c. On

Introduce page_to_bus() and use it in pci-dma-xen.c and swiotlb.c. On
xen/ia64 with the P2M/VP model pseudo physical address(gpaddr) is
fully virtualized so it defines
xen_features(XENFEAT_auto_translated_physmap) = 1.  In this case
page_to_phys(page) should return pseudo physical address like
pfn_to_mfn() and its families.  However dma is not virtualized, it
can't be used for pci-dma-xen.c, swiotlb.c.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>