qapi: introduce new cmd option "allow-oob"

qapi: introduce new cmd option "allow-oob"

Here "oob" stands for "Out-Of-Band".  When "allow-oob" is set, it means
the command allows out-of-band execution.

The "oob" idea is proposed by Markus Armbruster in following thread:

  https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg02057.html

This new "allow-oob" boolean will be exposed by "query-qmp-schema" as
well for command entries, so that QMP clients can know which commands
can be used in out-of-band calls. For example the command "migrate"
originally looks like:

  {"name": "migrate", "ret-type": "17", "meta-type": "command",
   "arg-type": "86"}

And it'll be changed into:

  {"name": "migrate", "ret-type": "17", "allow-oob": false,
   "meta-type": "command", "arg-type": "86"}

This patch only provides the QMP interface level changes.  It does not
contain the real out-of-band execution implementation yet.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-18-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: rebase on introspection done by qlit]
Signed-off-by: Eric Blake <eblake@redhat.com>