migration/postcopy: Update the bandwidth during postcopy

migration/postcopy: Update the bandwidth during postcopy

The recently added max-postcopy-bandwidth parameter is only read
at the transition from precopy->postcopy where as the older
max-bandwidth parameter updates the migration bandwidth when changed
even if the migration is already running.

Fix this discrepency so that:
  a) You can change the bandwidth during postcopy by setting
     max-postcopy-bandwidth

  b) Changing max-bandwidth during postcopy has no effect
     (it currently changes the postcopy bandwidth which isn't
     expected).

Fixes: 7e555c6c
bz: https://bugzilla.redhat.com/show_bug.cgi?id=1686321
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>