mmu: read page content before populating page table

Page should not be accessible through a page table before it contains
valid data, otherwise a thread running on another vcpu can see garbage.
We rely on zfs file prefetch mechanism to minimize actual disk IO for
mmap_populate mappings. In the feature this will be replaced with proper
page cache integration.

Fixes #228

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