vm: Fix object init for MPS allocation

Initialize object with placement new after MPS memory region has been
committed.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
vm: Make string a proper object

Make string a proper object so that class is initialized to something
reasonable and instructions like invokevirtual won't touch random
memory.

Signed-off-by: Pekka Enberg <penberg@iki.fi>