Flush already closed frames at plugin exit even at the cost of having no post-state for the final frame

[moste00]

** Problem **

QEMU plugin API provides no direct way to run a callback at the retiring of every instruction. This is necessary for us because we record post-state for each instruction.

The way the plugin deals with it is by treating the pre-execution boundary of an instruction as the post-excecution boundary of the previous instruction.

But this becomes a problem at the very end of execution, since the final instruction has no next instruction, so we have to capture its post-state at plugin exit, but this is impossible because there is no VCPU in the plugin exit callback.

** Previous State **

This was previously "solved" by just flushing a threshold (64) of frames periodically and accepting that the final batch of less-than-threshold frames will not be flushed.

** Solution **

The piqi format allows a frame with no-post state, so instead of sacrificing the entire residual array of frames at plugin exit, we can just sacrifice only the final frame and close it with no post-state, but flush the rest of the frames that were already closed.

[Rot127]

with no post-state

Have you tried if this goes well with rz-tracetest?