block-migration: limit the memory usage

If we set migration speed in a very large value, block-migration will try to read
all data to the memory. Because
    (block_mig_state.submitted + block_mig_state.read_done) * BLOCK_SIZE
will be overflow, and it will be always less than rate limit.

There is no need to read too many data into memory when the rate limit is very large.
So limit the memory usage can fix the overflow problem.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
