mmu: generic page table walker based on linear_map_level

This patch implements generic page table walker that traverse page table
levels in compile time. It accepts page_mapper class, that controls
various aspects of page traversing, as a parameter.  page_table_operation
controls whether non present intermediate page should be allocated,
how to handle leaf small/huge pages, whether to split huge pages, how to
handle sub area of a huge page in case splitting is disabled and whether
walker should loop over multiple page entries.

linear_map_level() is modified to use new page walker.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>