migration: Adjust postcopy bandwidth during switchover

migration: Adjust postcopy bandwidth during switchover

Precopy uses unlimited bandwidth always during switchover, it makes sense
because this is so critical and no one would like to throttle bandwidth
during the VM blackout.

OTOH, postcopy surprisingly didn't do that.  There's one line that in the
middle of the postcopy switchover it tries to switch to postcopy's
specified max-postcopy-bandwidth, but even so it's somewhere in the middle
which is strange.

This patch brings the two modes to always use unlimited bandwidth for
switchover, meanwhile only apply the postcopy max bandwidth after the
switchover is completed.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-8-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>