x86-64: use 1GB pages in 1:1 mapping if available

x86-64: use 1GB pages in 1:1 mapping if available

At once adjust the 2/4Mb page handling slightly in a few places (to
match the newly added code):
- when re-creating a large page mapping after finding that all small
  page mappings in the respective area are using identical flags and
  suitable MFNs, the virtual address was already incremented pas the
  area to be dealt with, which needs to be accounted for in the
  invocation of flush_area() in that path
- don't or-in/and-out _PAGE_PSE on non-present pages
- when comparing flags, try minimse the number of l1f_to_lNf()/
  lNf_to_l1f() instances used
- instead of skipping a single page when encountering a big page
  mapping equalling to what a small page mapping would establish, skip
  to the next larger page boundary

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