Extend the hypervisor API with name-only shutdown

Currently the ShutdownInstance method of the hypervisors takes a full
instance object. However, when doing instance shutdowns from the node
only, we don't have a full object, just the name.

To handle this use case, we add a new ‘name’ argument to the method,
which makes the shutdown not use/rely on the ‘instance’ argument. The
KVM and fake hypervisors need a little bit of work, otherwise the change
is straightforward.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>