Pretending that QMP doesn't understand a command merely because
we are not in the right mode doesn't help first-time users figure
out what to do to correct things.  Although the documentation for
QMP calls out capabilities negotiation, we should also make it
clear in our error messages what we were expecting.  With this
patch, I now get the following transcript:

$ ./x86_64-softmmu/qemu-system-x86_64 -qmp stdio -nodefaults
{"QMP": {"version": {"qemu": {"micro": 93, "minor": 2, "major": 2}, "package": ""}, "capabilities": []}}
{"execute":"huh"}
{"error": {"class": "CommandNotFound", "desc": "The command huh has not been found"}}
{"execute":"quit"}
{"error": {"class": "CommandNotFound", "desc": "Expecting capabilities negotiation with 'qmp_capabilities' before command 'quit'"}}
{"execute":"qmp_capabilities"}
{"return": {}}
{"execute":"qmp_capabilities"}
{"error": {"class": "CommandNotFound", "desc": "Capabilities negotiation is already complete, command 'qmp_capabilities' ignored"}}
{"execute":"quit"}
{"return": {}}
{"timestamp": {"seconds": 1429110729, "microseconds": 181935}, "event": "SHUTDOWN"}
Signed-off-by: NEric Blake <eblake@redhat.com>
Tested-By: NKashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: NPaulo Vital <paulo.vital@profitbricks.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>