trace/simple: Allow enabling simple traces from command line

trace/simple: Allow enabling simple traces from command line

The simple trace backend is enabled / disabled with a call
to st_set_trace_file_enabled().  When initializing tracing
from the command-line, this must be enabled on startup.
(Prior to db25d56c014aa1a9, command-line initialization of
simple trace worked because every call to st_set_trace_file
enabled tracing.)

Fixes: db25d56c014aa1a96319c663e0a60346a223b31e
Signed-off-by: Josh DuBois <josh@joshdubois.com>
Message-id: 20200723053359.256928-1-josh@joshdubois.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>