hmp: human-monitor-command: stop using the Memory chardev driver

The Memory chardev driver was added because, as the Monitor's output
buffer was static, we needed a way to accumulate the output of an
HMP commmand when ran by human-monitor-command.

However, the Monitor's output buffer is now dynamic, so it's possible
for the human-monitor-command to use it instead of the Memory chardev
driver.

This commit does that change, but there are two important
observations about it:

 1. We need a way to signal to the Monitor that it shouldn't call
    chardev functions when flushing its output. This is done
    by adding a new flag to the Monitor object called skip_flush
	(which is set to true by qmp_human_monitor_command())

 2. The current code has buffered semantics: QMP clients will
    only see a command's output if it flushes its output with
	a new-line character. This commit changes this to unbuffered,
	which means that QMP clients will see a command's output
	whenever the command prints anything.

	I don't think this will matter in practice though, as I believe
	all HMP commands print the new-line character anyway.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>