monitor: resurrect handle_qmp_command trace event

Commit 104fc302 ("qmp: Drop duplicated
QMP command object checks") removed the call to
trace_handle_qmp_command() while eliminating code duplication.

This patch brings the trace event back so QEMU-internal trace events can
be correlated with the QMP commands that caused them.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170605104216.22429-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>