Feature negotiation allows clients to enable new QMP capabilities they
support and thus allows QMP to envolve in a compatible way.
A capability is a new QMP feature and/or protocol change which is not part of
the core protocol as defined in the QMP spec.
Feature negotiation is implemented by, among other changes, adding
mode-oriented support to QMP, which comprehends the following:
o Two modes: handshake and operational
o All QMP Monitors start in handshake mode
o In handshake mode only commands to query/enable/disable QMP capabilities are
allowed (there are few exceptions)
o Clients can switch to the operational mode at any time
o In Operational mode most commands are allowed and QMP capabilities changes
made in handshake mode take effect
Please, note that we don't have any capability yet. So, the most visable
change in this series is that now Clients must switch to operational mode to
be able to issue 'regular' commands.
Session example:
"""
{"QMP": {"capabilities": []}}
{ "execute": "query-qmp-mode" }
{"return": {"mode": "handshake"}}
{ "execute": "stop" }
{"error": {"class": "CommandNotFound", "desc": "The command stop has not been
found", "data": {"name": "stop"}}}
{ "execute": "qmp_capability_enable", "arguments": { "name": "foobar" } }
{"error": {"class": "InvalidParameter", "desc": "Invalid parameter name",
"data": {"name": "name"}}}
{ "execute": "qmp_switch_mode", "arguments": { "mode": "operational" } }
{"return": {}}
{ "execute": "query-qmp-mode" }
{"return": {"mode": "operational"}}
{ "execute": "stop" }
{"return": {}}
"""
