qemu.bbclass: fix a python TypeError

qemu.bbclass: fix a python TypeError

QEMU_OPTIONS can be empty which will trigger a exception TypeError:
| can only concatenate str (not "NoneType") to str. Fix it by setting a
empty string.

ALso removed two useless blanks.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b619197bd52a4a99a9989e7ea6fb7032415b1e42)
Signed-off-by: Steve Sakoman <steve@sakoman.com>