char-stdio: Fix QMP default for 'signal'

char-stdio: Fix QMP default for 'signal'

Commit 02c4bdf1 tried to make signal=on the default for stdio chardevs
except for '-serial mon:stdio', but it forgot about QMP and accidentally
switched the QMP default from true (except for -nographic) to false
(always). The documentation was kept unchanged and still describes the
opposite of the old behaviour (which is an even older documentation
bug).

Fix all of this by making signal=true the default in ChardevStdio and
documenting it as such.

Fixes: 02c4bdf1d2ca8c02a9bae16398f260b5c08d08bf
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201023101222.250147-2-kwolf@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>