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

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: [email protected]
Signed-off-by: Paolo Bonzini <[email protected]>
Message-Id: <20210209145759[email protected]>