qmp: Drop duplicated QMP command object checks

qmp_check_input_obj() duplicates qmp_dispatch_check_obj(), except the
latter screws up an error message.  handle_qmp_command() runs first
the former, then the latter via qmp_dispatch(), masking the screwup.

qemu-ga also masks the screwup, because it also duplicates checks,
just differently.

qmp_check_input_obj() exists because handle_qmp_command() needs to
examine the command before dispatching it.  The previous commit got
rid of this need, except for a tracepoint, and a bit of "id" code that
relies on qdict not being null.

Fix up the error message in qmp_dispatch_check_obj(), drop
qmp_check_input_obj() and the tracepoint.  Protect the "id" code with
a conditional.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1488544368-30622-9-git-send-email-armbru@redhat.com>