[shimV2] add plan9 device controller

[rawahars]

Summary

This change adds the plan9 device controller which can add/remove plan9 shares from a VM. The guest side operations are part of mount controller responsibility.

Whenever a new request for AddToVM comes, we call into HCS to hot-add the path via HcsModifyComputeSystem, where you inject a Plan9Share entry into the VM’s Devices → Plan9 → Shares schema, and HCS plumbs the corresponding endpoints so the guest can mount it using the Plan 9 (9P) protocol.

Note

For the same host path, we add a new share to the UVM. This pattern is similar to the existing pattern.
Ref- https://github.com/microsoft/hcsshim/blob/87708ff3150b7bceca4dbb3f7cdb7147428e42c3/internal/uvm/plan9.go

Imagine that the caller wants to add the same host path at two different guest locations-

Location 1: /tools
Location2: /tools2

If we do not create a new share for the UVM, we are likely to send a second GCS request asking the same share to be mounted at tools2. When the guest receives the GuestRequest of type ResourceTypeMappedDirectory, the handler unconditionally calls plan9.Mount(). This will, regardless of whether the same share name / aname was already mounted, dials a brand new vsock connection to the Plan9 server, opens a file descriptor from it, and performs a fresh unix.Mount(..., "9p", ...) syscall. This leads to error.

If we need to perform refCounting for plan9, it would need changes on both shim and guest side.