migration/postcopy: Add latency distribution report for blocktime

migration/postcopy: Add latency distribution report for blocktime

Add the latency distribution too for blocktime, using order-of-two buckets.
It accounts for all the faults, from either vCPU or non-vCPU threads.  With
prior rework, it's very easy to achieve by adding an array to account for
faults in each buckets.

Sample output for HMP (while for QMP it's simply an array):

Postcopy Latency Distribution:
  [     1 us -     2 us ]:          0
  [     2 us -     4 us ]:          0
  [     4 us -     8 us ]:          1
  [     8 us -    16 us ]:          2
  [    16 us -    32 us ]:          2
  [    32 us -    64 us ]:          3
  [    64 us -   128 us ]:      10169
  [   128 us -   256 us ]:      50151
  [   256 us -   512 us ]:      12876
  [   512 us -     1 ms ]:         97
  [     1 ms -     2 ms ]:         42
  [     2 ms -     4 ms ]:         44
  [     4 ms -     8 ms ]:         93
  [     8 ms -    16 ms ]:        138
  [    16 ms -    32 ms ]:          0
  [    32 ms -    65 ms ]:          0
  [    65 ms -   131 ms ]:          0
  [   131 ms -   262 ms ]:          0
  [   262 ms -   524 ms ]:          0
  [   524 ms -    1 sec ]:          0
  [    1 sec -    2 sec ]:          0
  [    2 sec -    4 sec ]:          0
  [    4 sec -    8 sec ]:          0
  [    8 sec -   16 sec ]:          0

Cc: Markus Armbruster <armbru@redhat.com>
Acked-by: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250613141217.474825-15-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>