qmp: remove qmp_cpu

qmp: remove qmp_cpu

'qmp_cpu' was implemented in commit 755f196898 ("qapi: Convert the cpu
command") as a functional no-op, a QMP call that does nothing and
return success. The idea, apparently, was to provide a counterpart
for the HMP 'hmp_cpu' command, introduced in the same commit.

After 6 years of its creation, qmp_cpu remains a functional no-op
that does nothing, having no value for any caller/user. A proposal
was sent to implement qmp_cpu like hmp_cpu works, but it was denied
[1]. The reason is that QMP must be as stateless as possible and a
function that changes its state (the current CPU monitor in the case
of qmp_cpu) goes against it. Any QMP command that needs a specific
monitor CPU setup must provide it in its arguments, instead of relying
in the current QMP monitor state.

After discussions that happened in [2] it was decided that a command
that does nothing since its birth, no one uses for anything and will
not be implemented, should be deprecated and erased. Given that we will
*not* provide any replacement for qmp_cpu and we believe that there
is no user relying on it, there is no point in adding a deprecation
delay for it.

So, this patch nukes qmp_cpu from QEMU code, removing both its blank
implementation in qmp.c and its doc in qapi-schema.json.

[1] https://lists.gnu.org/archive/html/qemu-devel/2017-12/msg02283.html
[2] https://lists.gnu.org/archive/html/qemu-devel/2017-12/msg03696.html

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20171220102304.8288-1-danielhb@linux.vnet.ibm.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>