vgsp: raise the GSP engine event interrupt so guest ISR drains events and wakes poll() waiters

The emulated GSP only signalled the MSI-X eventfd when it queued a status
record. The guest's primary ISR (nvidia_isr) decides pending from the
CPU_INTR leaf registers, which we never set for the GSP stall vector, so it
returned IRQ_NONE: IRQ 33 counted thousands of interrupts while the nvidia
event thread sat idle and a client blocked in poll() on /dev/nvidia0 (a GLX
swap) waited forever even though the event was already in the status queue.

Mirror what a real GSP-RM does when it queues an event:
  - program RISC-V IRQMASK/IRQDEST for SWGEN0 (read-only to the guest; the
    GSP firmware owns them),
  - set SWGEN0 in the falcon IRQSTAT,
  - latch GSP stall vector 155 in the CPU interrupt tree (LEAF[4] bit 27,
    UVM_SHARED subtree 2 -> TOP[0] bit 2),
  - signal MSI-X.

Raise it only for POST_EVENT records (replies are polled by the client).
Verified on r1680: the guest now writes IRQSCLR SWGEN0, kgspService_TU102
drains events, and glxgears renders (was a hard hang).

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