Multiple Plan9 mounts to the same files are not cache coherent

[jstarks]

See #463.

This may cause issues in some POD scenarios.

[jterry75]

I assume the correct fix here is to make a refcount in the host and a refcount in the guest. IE: we might have a single share to a single UVM location but then 2x bind mounts from that single UVM location to a container location. Does this seem like overkill?

[jstarks]

I think the cleanest fix is to add refcounting in the hcsoci layer instead of trying to hide it in the uvm code. I don't think the guest cares.