Rapi: read and use the vm_capable node flag

Similar to the IAllocator change, this patch reads and uses the
vm_capable flag in Rapi. Furthermore, it changes the group UUID
reading to the same maybeFromObj infrastructure.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>