Fix redistribution of files w.r.t. offline nodes

Currently, _RedistributeAncillaryFiles computes two lists: the list of
online nodes (for all files redistribution), and the list of
vm_capable nodes, for hypervisor-specific files. However, the
vm_capable list includes offline nodes too, leading to warning
messages:

  WARNING: Copy of file /etc/xen/xend-config.sxp to node node13.example.com failed: Node is marked offline

We fix this by trivially intersecting the vm_capable list with the
online one.

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