balloon: Fix documentation of the --balloon parameter and deprecate it

There are two issues with the documentation of the --balloon parameter:
First, "--balloon none" is simply doing nothing. Even if a machine had a
balloon device by default, this option is not disabling anything, it is
simply ignored. Thus let's simply drop this option from the documentation
to avoid to confuse the users (but keep the code in vl.c for backward
compatibility).
Second, the documentation claims that "--balloon virtio" is the default
mode, but this is not true anymore since commit 382f0743.
Since that commit, the option also has no real use case anymore, since
you can simply use "--device virtio-balloon" nowadays instead. Thus to
simplify our complex parameter zoo a little bit, let's deprecate the
the parameter now and tell the user to use "--device virtio-balloon"
instead.

Fixes: 382f0743
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1519796303-13257-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>