hmp: Disable chardev-add and chardev-remove

As a general rule, HMP commands must be built on top of the QMP API.
Luiz and others have worked long & hard to make HMP conform to this
rule.

Commit f1088908 added chardev-add, in violation of this rule.  QMP
command chardev-add was added right before, with minimal features, and
the idea to complete it step by step, then switch over the HMP command
to use it.

Unfortunately, we're not there, yet, and we don't want to release with
chardev-add in a "HMP is more powerful than QMP" state.

Disable the HMP command for now, along with its chardev-remove buddy.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>