Description of problem:
via qmp or hmp command, found the default value of migration max-postcopy-bandwidth is 0, and no unit


Version-Release number of selected component (if applicable):
host info:
src host: kernel-4.18.0-80.5.el8.x86_64 & qemu-img-3.1.0-20.module+el8+2904+e658c755.x86_64
dst host: kernel-4.18.0-80.el8.x86_64 & qemu-img-3.1.0-20.module+el8+2904+e658c755.x86_64
guest info:
kernel-4.18.0-73.el8.x86_64


How reproducible:
100%


Steps to Reproduce:
1.boot guest on src host
2.via hmp or qmp, check max-postcopy-bandwidth default value:
(1) hmp 
(qemu) info migrate_parameters 
...
max-bandwidth: 33554432 bytes/second
downtime-limit: 300 milliseconds
x-checkpoint-delay: 20000
block-incremental: off
x-multifd-channels: 2
x-multifd-page-count: 16
xbzrle-cache-size: 67108864
max-postcopy-bandwidth: 0
(2)qmp
{"execute":"query-migrate-parameters"}
{"return": {"xbzrle-cache-size": 67108864, "cpu-throttle-initial": 20, "decompress-threads": 2, "compress-threads": 8, "compress-level": 1, "block-incremental": false, "compress-wait-thread": true, "downtime-limit": 300, "tls-creds": "", "x-multifd-page-count": 16, "max-cpu-throttle": 99, "max-postcopy-bandwidth": 0, "tls-hostname": "", "x-multifd-channels": 2, "max-bandwidth": 33554432, "x-checkpoint-delay": 20000, "cpu-throttle-increment": 10}}


Actual results:
like above step 2


Expected results:
from step 2
(1)the default value of migration max-postcopy-bandwidth should not be 0
(2)via hmp command, there no unit about the value of max-postcopy-bandwidth, should like max-bandwidth, has unit such as "bytes/second"


Additional info:
(1)when use the default max-postcopy-bandwidth value "0" to migrate, found throughput is not 0 after migration finished:
(qemu) info migrate
globals:
...
Migration status: completed
total time: 18789 milliseconds
downtime: 22 milliseconds
setup: 19 milliseconds
transferred ram: 1974360 kbytes
throughput: 861.76 mbps