migration/ram: Add outgoing 'mapped-ram' migration

migration/ram: Add outgoing 'mapped-ram' migration

Implement the outgoing migration side for the 'mapped-ram' capability.

A bitmap is introduced to track which pages have been written in the
migration file. Pages are written at a fixed location for every
ramblock. Zero pages are ignored as they'd be zero in the destination
migration as well.

The migration stream is altered to put the dirty pages for a ramblock
after its header instead of having a sequential stream of pages that
follow the ramblock headers.

Without mapped-ram (current):        With mapped-ram (new):

 ---------------------               --------------------------------
 | ramblock 1 header |               | ramblock 1 header            |
 ---------------------               --------------------------------
 | ramblock 2 header |               | ramblock 1 mapped-ram header |
 ---------------------               --------------------------------
 | ...               |               | padding to next 1MB boundary |
 ---------------------               | ...                          |
 | ramblock n header |               --------------------------------
 ---------------------               | ramblock 1 pages             |
 | RAM_SAVE_FLAG_EOS |               | ...                          |
 ---------------------               --------------------------------
 | stream of pages   |               | ramblock 2 header            |
 | (iter 1)          |               --------------------------------
 | ...               |               | ramblock 2 mapped-ram header |
 ---------------------               --------------------------------
 | RAM_SAVE_FLAG_EOS |               | padding to next 1MB boundary |
 ---------------------               | ...                          |
 | stream of pages   |               --------------------------------
 | (iter 2)          |               | ramblock 2 pages             |
 | ...               |               | ...                          |
 ---------------------               --------------------------------
 | ...               |               | ...                          |
 ---------------------               --------------------------------
                                     | RAM_SAVE_FLAG_EOS            |
                                     --------------------------------
                                     | ...                          |
                                     --------------------------------

where:
 - ramblock header: the generic information for a ramblock, such as
   idstr, used_len, etc.

 - ramblock mapped-ram header: the new information added by this
   feature: bitmap of pages written, bitmap size and offset of pages
   in the migration file.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240229153017.2221-10-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>