qmp: Revert change to handle_qmp_command tracepoint

qmp: Revert change to handle_qmp_command tracepoint

Commit 71da4667db6 "monitor: separate QMP parser and dispatcher" moved
the handle_qmp_command tracepoint from handle_qmp_command() to
monitor_qmp_dispatch_one().  This delays tracing from enqueue time to
dequeue time.  Revert that.  Dequeue remains adequately visible via
tracepoint monitor_qmp_cmd_in_band.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-14-armbru@redhat.com>