Add support for vm_capable in cluster verify

The method to make vm_capable integrate easily into cluster verify is as follows:

- we add a new NV_VMNODES that represents *non*-vm-capable nodes
- the LU populates this list (it's expected that non-vm_capable nodes
  are few compared to vm_capable nodes)
- backend skips the checks that are related to VM hosting
- in the LU, we reorder the VM-related checks so that they occur after
  the non-VM (generic) tests, and we only execute them conditionally

Additionally, we add some support to the instance checks to detect
instances living on bad nodes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>