docs/devel: document expectations for HMP commands in the future

docs/devel: document expectations for HMP commands in the future

We no longer wish to have commands implemented in HMP only. All commands
should start with a QMP implementation and the HMP merely be a shim
around this. To reduce the burden of implementing QMP commands where
there is low expectation of machine usage, requirements for QAPI
modelling are relaxed provided the command is under the "x-" name
prefix.

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