monitor: add handle_hmp_command trace event

It is often useful to correlate QEMU-internal events with monitor
commands that caused them.  Trace the full HMP command being executed.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170605104216.22429-2-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>