migration: introduce pages-per-second

migration: introduce pages-per-second

It introduces a new statistic, pages-per-second, as bandwidth or mbps is
not enough to measure the performance of posting pages out as we have
compression, xbzrle, which can significantly reduce the amount of the
data size, instead, pages-per-second is the one we want

Signed-off-by: Xiao Guangrong <[email protected]>
Message-Id: <20190111063732[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
  With typo's Eric spotted fixed