migration/dirtyrate: Show sample pages only in page-sampling mode

migration/dirtyrate: Show sample pages only in page-sampling mode

The value of "Sample Pages" is confusing in mode other than page-sampling.
See below:

(qemu) calc_dirty_rate -b 10 520
(qemu) info dirty_rate
Status: measuring
Start Time: 11646834 (ms)
Sample Pages: 520 (per GB)
Period: 10 (sec)
Mode: dirty-bitmap
Dirty rate: (not ready)

(qemu) info dirty_rate
Status: measured
Start Time: 11646834 (ms)
Sample Pages: 0 (per GB)
Period: 10 (sec)
Mode: dirty-bitmap
Dirty rate: 2 (MB/s)

While it's totally useless in dirty-ring and dirty-bitmap mode, fix to
show it only in page-sampling mode.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>