ram: refactor ram_save_block() return value

ram: refactor ram_save_block() return value

It could only return 0 if we only found dirty xbzrle pages that hadn't
changed (i.e. they were written with the same content).  We don't care
about that case, it is the same than nothing dirty.

So now the return of the function is how much have it written, nothing
else. Adjust callers.

And we also made ram_save_iterate() return the number of transferred
bytes, not the number of transferred pages.

Signed-off-by: Juan Quintela <quintela@redhat.com>