Add a qemuMonitorGetCommands() method for QMP query-commands command

Add a qemuMonitorGetCommands() method for QMP query-commands command

Add a new qemuMonitorGetCPUCommands() method to support invocation
of the 'query-commands' JSON monitor command. No HMP equivalent
is required, since this will only be used when JSON is available

The existing qemuMonitorJSONCheckCommands() method is refactored
to use this new method

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>