hmp: remove "change vnc TARGET" command

hmp: remove "change vnc TARGET" command

The HMP command \"change vnc TARGET\" is messy:

- it takes an ugly shortcut to determine if the option has an "id",
with incorrect results if "id=" is not preceded by an unescaped
comma.

- it deletes the existing QemuOpts and does not try to rollback
if the parsing fails (which is not causing problems, but only due to
how VNC options are parsed)

- because it uses the same parsing function as "-vnc", it forces
the latter to not support "-vnc help".

On top of this, it uses a deprecated QMP command, thus getting in
the way of removing the QMP command.  Since the usecase for the
command is not clear, just remove it and send "change vnc password"
directly to the QMP "change-vnc-password" command.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20210120144235.345983-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>