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 <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>