NULL is a valid log filename used to indicate we want to use stderr
but qemu_set_log_filename (which is called by bsd-user/main.c) was not
handling it correctly.

That also made redundant a couple of NULL checks in calling code which
have been removed.

Signed-off-by: Salvador Fandino <salvador@qindel.com>
Message-Id: <20200123193626.19956-1-salvador@qindel.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>