app: fix osv::application never getting destroyed

Fixes #434.

The 'app' variable never went out of scope causing shared_ptr to
remain live.

Tested-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>