monitor: Reduce handle_qmp_command() tracing overhead

monitor: Reduce handle_qmp_command() tracing overhead

We are malloc'ing a QString and spending CPU cycles on converting a
QObject to string, just for the sake of sticking the string in the trace
message.  Wasted when we aren't tracing.  Avoid that.

[Commit message and description suggested by Markus Armbruster to
provide more detail about the rationale for this patch.

Use trace_event_get_state_backends() instead of trace_event_get_state()
to honor DTrace/UST backend dstates.
--Stefan]

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170725143923.11241-1-den@openvz.org
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Lluís Vilanova <vilanova@ac.upc.edu>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>