vl: fix tracing initialization

we should call trace_init_backends() before trace_init_file() for
CONFIG_TRACE_SIMPLE There is no difference for other cases.

This problem was introduced by the commit
    commit 41fc57e44ed64cd4ab5393d83624afd897dabd4f
    Author: Paolo Bonzini <pbonzini@redhat.com>
    Date:   Thu Jan 7 16:55:24 2016 +0300

    trace: split trace_init_file out of trace_init_backends

'make check' was failed as a result if configured with
  --enable-trace-backends=simple

Spotted by Alex Bennée.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-id: 1455036545-14870-1-git-send-email-den@openvz.org
CC: Alex Bennée <alex.bennee@linaro.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
