Introduce new flavour of map_domain_page()

Introduce new flavour of map_domain_page()

Introduce a variant of map_domain_page() directly getting passed a
struct page_info * argument, based on the observation that in many
places the argument to this function so far simply was the result of
page_to_mfn(). This is meaningful for the x86-64 case where
map_domain_page() really just is an invocation of mfn_to_virt(), and
hence the combined mfn_to_virt(page_to_mfn()) now represents a
needless round trip conversion compressed -> uncompressed ->
compressed of the MFN representation.

Signed-off-by: Jan Beulich <jbeulich@novell.com>