package/qemu: fix specifying custom target sub-set

The logic was wrong.

Even though it was working for previous versions of QEMU, it changed in
later versions, and thus now breaks on the version we currently package.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>