controller/vm: add unit tests for VM Controller state machine

Defines per-platform vmLifetime and guestManager seam interfaces over
*vmmanager.UtilityVM and *guestmanager.Guest. Each interface is a superset
of the methods Controller invokes directly plus the device methods consumed
by the wired-in sub-controllers (vpci/network/plan9/scsi). *vmmanager and
*guestmanager satisfy these via Go structural typing; no concrete sister
fields needed.

The per-platform split is necessary because:
  - AddPlan9 / RemovePlan9 are LCOW-only on *vmmanager.UtilityVM.
  - Guest network signatures differ across LCOW and WCOW.
  - vPCI guest and Plan9 guest methods are LCOW-only.
  - UpdateHvSocketAddress is WCOW-only.

Guest() uses a type assertion to preserve its concrete return type.

30 tests covering:

Idempotency (3): duplicate StartVM no-op, TerminateVM from Terminated
  and NotCreated.

State guards (8): StartVM rejects NotCreated/Terminated/Invalid,
  ExecIntoHost rejects Terminated, terminal+stderr input validation,
  Update methods reject non-Running (4 subtests), Stats rejects
  Terminated, Wait rejects NotCreated, DumpStacks rejects Terminated,
  UpdateCPUGroup rejects empty ID.

TerminateVM cleanup chain (6): full Terminate->CloseConnection->Close
  ordering, guest close failure swallowed, uvm.Terminate failure
  swallowed, uvm.Close failure -> StateInvalid, recovery from Invalid,
  retry failure stays Invalid.

Error propagation (4): ExecIntoHost guest error, DumpStacks guest error,
  UpdatePolicyFragment guest error, ExitStatus not-terminated error.

API delegation (4): ExecIntoHost exit code forwarding, DumpStacks
  with and without capability, ExitStatus value assembly.

Wait + background goroutine (5): Wait from Terminated with log drain,
  Wait context cancelled during log drain, Wait Running happy path,
  waitForVMExit Running->Terminated transition, waitForVMExit
  overwrites Invalid->Terminated (pins current behaviour).

Mocks generated per-platform (mocks/mock_lcow_vm.go, mock_wcow_vm.go)
with matching //go:build constraints.

Signed-off-by: Shreyansh Sancheti <shsancheti@microsoft.com>

Author: Shreyansh Sancheti