core: Add support for aligned page allocation

core: Add support for aligned page allocation

Refactor page_alloc to page_alloc_internal which accepts an additional
constraint for its allocation: align_mask. The allocated region will now
have its start page chosen so that page_number & align_mask is zero. If
no alignment is required, align_mask just needs to be set to 0. This is
what page_alloc exploits.

However, the new function page_alloc_aligned is introduces to return
page regions aligned according to their size (num pages will be aligned
by num * PAGE_SIZE). This implied that num needs to be a power of two.

This will be used on the AArch64 port of Jailhouse to support physical
address ranges from 40 to 44 bits: in these configurations, the initial
page table level may take up multiple consecutive pages.

Based on patch by Antonios Motakis.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>