MSRIOGroup: File based save and restore does not use batch context tracking

[bgeltz]

The ability to maintain multiple batch contexts for MSRIO was introduced in PR #2972. This PR also modified the MSRIOGroup's in-memory version of save_control() to leverage the new batch context tracking by introducing m_save_restore_ctx. This version of save_control() is only ever called by the Controller during Controller::run(). This call will only ever get into the MSRIOGroup if the end-user has direct access to the MSR or msr-safe file descriptors (which is only possible presently if the user has CAP_SYS_ADMIN).

During "normal" expected operation where the end-user does not have direct access to manipulate MSRs, all of that I/O happens through the ServiceIOGroup instead. When the user initiates a new session, and that session is a write_mode() session, pio.save_control_dir() is invoked from service.py. This API ultimately calls into the file-based version of MSRIOGroup::save_control(const std::string path). This file-based version utilizes the SaveControl object which ultimately uses read_signal() to determine the value to "save" initially.

This all became apparent through the investigation of #3352.

The following needs to be changed in order for SaveControl to utilize the batch context tracking:

• SaveControl needs to have a way to issue read_batch and sample either through the existing m_save_restore_ctx or a new one. Presently in MSRIOGroup, this is done through direct access to the MSRFieldControl objects that are contained within m_control_available. The MSRFieldControl objects have a save() method which calls sample() and stores the result in a class member. This could somehow be exposed for SaveControl to query. This could be done by making SaveControl a friend of MSRIOGroup, and making m_control_available protected instead of private.
• A better solution would be to plumb the batch context through the IOGroup interface so that this wouldn't be a one-off solution just for the MSRIOGroup.

This is a good topic for an architecture meeting as multiple solutions are possible.