migration: add RAMBlock field and helper for fast snapshot load

Add pending_bmap field per RAMBlock which is a Bitmap to store
internal state of which pages have been read by some thread to ensure
coordination between fault thread and eager load thread.

Modify parse_ramblock_mapped_ram(), to not load the actual RAMBlocks
data in postcopy case as that will be loaded by fault thread and eager
thread after the VM starts running.

Change ram_load() to use new function ram_should_load_postcopy_pages()
to decide how to load/read RAM.

Signed-off-by: Aadeshveer Singh <aadeshveer07@gmail.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>