util/vfio-helpers: Use error_setg in qemu_vfio_find_[fixed/temp]_iova

util/vfio-helpers: Use error_setg in qemu_vfio_find_[fixed/temp]_iova

Both qemu_vfio_find_fixed_iova() and qemu_vfio_find_temp_iova()
return an errno which is unused (or overwritten). Have them propagate
eventual errors to callers, returning a boolean (which is what the
Error API recommends, see commit e3fe3988d78 "error: Document Error
API usage rules" for rationale).

Suggested-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20210902070025.197072-9-philmd@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>