linux-user: init_guest_space: Clean up if we can't initialize the commpage

We'll just exit with an error anyway, so it doesn't really matter, but it
is cleaned up in all of the other places were we error out.

Signed-off-by: Luke Shumaker <lukeshu@parabola.nu>
Message-Id: <20171228180814.9749-4-lukeshu@lukeshu.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>