docs/devel: document expectations for QAPI data modelling for QMP

docs/devel: document expectations for QAPI data modelling for QMP

Traditionally we have required that newly added QMP commands will model
any returned data using fine grained QAPI types. This is good for
commands that are intended to be consumed by machines, where clear data
representation is very important. Commands that don't satisfy this have
generally been added to HMP only.

In effect the decision of whether to add a new command to QMP vs HMP has
been used as a proxy for the decision of whether the cost of designing a
fine grained QAPI type is justified by the potential benefits.

As a result the commands present in QMP and HMP are non-overlapping
sets, although HMP comamnds can be accessed indirectly via the QMP
command 'human-monitor-command'.

One of the downsides of 'human-monitor-command' is that the QEMU monitor
APIs remain tied into various internal parts of the QEMU code. For
example any exclusively HMP command will need to use 'monitor_printf'
to get data out. It would be desirable to be able to fully isolate the
monitor implementation from QEMU internals, however, this is only
possible if all commands are exclusively based on QAPI with direct
QMP exposure.

The way to achieve this desired end goal is to finese the requirements
for QMP command design. For cases where the output of a command is only
intended for human consumption, it is reasonable to want to simplify
the implementation by returning a plain string containing formatted
data instead of designing a fine grained QAPI data type. This can be
permitted if-and-only-if the command is exposed under the 'x-' name
prefix. This indicates that the command data format is liable to
future change and that it is not following QAPI design best practice.

The poster child example for this would be the 'info registers' HMP
command which returns printf formatted data representing CPU state.
This information varies enourmously across target architectures and
changes relatively frequently as new CPU features are implemented.
It is there as debugging data for human operators, and any machine
usage would treat it as an opaque blob. It is thus reasonable to
expose this in QMP as 'x-query-registers' returning a 'str' field.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>