console: make QMP screendump use coroutine Hi, Thanks to recent work by Kevin, it becomes possible to run HMP/QMP commands in a coroutine. The screendump command is a good target, as it requires to re-enter the main-loop in ordre to flush the display, and write to file. Ideally, IO would be done with a non-blocking fd, however it's not currently enabled. This is left for a future work. v2: - change summary to not falsly claim non-blocking write support - code styles fixes - rebased, add reviewed-by tags