Fix QEMU_WARN_UNUSED_RESULT

Fix QEMU_WARN_UNUSED_RESULT

Since commit 747bbdf7 QEMU_WARN_UNUSED_RESULT is never defined as it is
conditional on a define from config-host.h which is included only later.
Include that file earlier to get the warnings back.

Reactivating it unfortunately leads to some warnings about unused qdev_init
results. These calls are changed to qdev_init_nofail to avoid build failures.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit beb6f0de7aa19ce62fdd21a876b8c171ac7fde2e)