With active mode, the guest write speed is limited by the synchronous
writes to the mirror target. For this reason, management applications
might want to start out in background mode and only switch to active
mode later, when certain conditions are met. This series adds a
block-job-change QMP command to acheive that, as well as
job-type-specific information when querying block jobs, which
can be used to decide when the switch should happen.
For now, only the direction background -> active is supported.
The information added upon querying is whether the target is actively
synced, the total data sent, and the remaining dirty bytes.
Initially, I tried to go for a more general 'job-change' command, but
I couldn't figure out a way to avoid mutual inclusion between
block-core.json and job.json.