mmu: Make map_file() more efficient

Currently map_file() do three passes over vma memory in a worst case.
First it maps memory with write permission while zeroing it, then it
reads a file into memory and, if vma is read only, it does one more
pass to fix memory permissions. Fix it by providing new specialization
of fill_page class which builds iovec of all allocated memory and
reads from a file using the iovec at the end of populate stage.

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