hw/arm/virt: Fix memory leak reported by Coverity

As the conditional statement had to be split anyway, we can also
add a better error report message.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-id: 1426877982-3603-1-git-send-email-sw@weilnetz.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
