ram: Split dirty bitmap by RAMBlock

Both the ram bitmap and the unsent bitmap are split by RAMBlock.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Peter Xu <peterx@redhat.com>

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Fix compilation when DEBUG_POSTCOPY is enabled (thanks Hailiang)