mmu: separate page allocation from page mapping process

Currently pages are allocated just before they are mapped, but sometimes
we want to map preallocated pages (for shared mapping or mapping page
cache page for instance). This patch moves page acquisition process into
separate class. This will allow us to add subclass that provides pages
from shared pages pool or from a page cache.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>