trace: skip qemu_set_log_filename if no "-D" option was passed

When the "simple" backend is not active but the "log" backend is,
both "-trace file=" and "-D" will result in a call to
qemu_set_log_filename.  Unfortunately, QEMU was also calling
qemu_set_log_filename if "-D" was not passed, so the "-trace
file=" option had no effect and the tracepoints went back to
stderr.

Fortunately we can just skip qemu_set_log_filename in that case,
because the log backend will initialize itself just fine as soon
as qemu_set_log is called, also in qemu_process_early_options.

Cc: stefanha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210209145759.141231-3-pbonzini@redhat.com>

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