tests/migration/postcopy: trim migration bandwidth

tests/migration/postcopy: trim migration bandwidth

On slow hosts with tcg we were sometimes finding that the migration
would complete during precopy and never get into the postcopy test.
Trim back the bandwidth a bit to make that much less likely.

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <20190923131022[email protected]>
Reviewed-by: Cleber Rosa <[email protected]>
Acked-by: Alex Bennée <[email protected]>
Tested-by: Cleber Rosa <[email protected]>
Tested-by: Alex Bennée <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>