With this commit QMP will be started in Capabilities Negotiation
mode, where the only command allowed to run is 'qmp_capabilities'.

All other commands will return CommandNotFound error. Asynchronous
messages are not delivered either.

When 'qmp_capabilities' is successfully executed QMP enters in
Command mode, where all commands (except 'qmp_capabilities') are
allowed to run and asynchronous messages are delivered.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>