kernel: Fix guest-hang root cause (VFIO mmap garbage PFN) + rebuild mdev remove path

VFIO mmap (main.c): virt_to_phys() was used on vmalloc-family addresses
(vzalloc'd BAR0 shadow, ioremap'd BAR2/3), producing garbage PFNs. Guest
BAR0 access then hit a KVM make_spte reserved-bits WARN and an
EPT-violation livelock, hard-wedging the guest on every nvidia driver
probe (confirmed via QEMU GDB stub). BAR0 now maps per-page via
vmalloc_to_pfn(); BAR2/3/4/5 map bars[N].gpa directly (like BAR1);
config space uses new nv_vgpu_mmap_virt_to_pfn() helper.

mdev remove path (main.c): old path never list_del'd the device
(available_instances leaked), never ran cleanup (leaked gsp_emu kthread),
never unregistered VFIO (leaked /dev/vfio/N + iommu group), never put
pdev, never freed the struct. New kvmgt-style path: cancel init_work ->
wait fds -> vfio_unregister_group_dev -> cleanup work -> list_del ->
pci_dev_put -> vfio_put_device.

gsp_rpc_client.c: fix dma_map_page dma_addr_t->int truncation in
gsp_rpc_alloc_emul (corrupts >=4GB IOVAs); export gsp_client_init_emulation.

gsp_emulation.c: export gsp_emu_process_rpc and gsp_emu_get_stats.

main.c: remove wrong MODULE_SOFTDEP("pre: nvidia") - module has zero
nvidia imports, loads standalone, and must load BEFORE nvidia.ko
(it exports nvidia_vgpu_vfio_* for nvidia.ko to import).

nvidia-vgpu-vfio.h: NULL-safe nv_vgpu_{dbg,info,warn,err} macros.

tests/gsp_emulation_test.c: rewritten against current API - drives
gsp_emu_process_rpc() with crafted shared-memory messages (protocol
validation, SCHEDULE/YIELD/NOTIFY/DEBUG dispatch, stress). Builds with
KBUILD_EXTRA_SYMBOLS; depends: open-iov-vfio.

stack_manager.sh: vfio_mdev doesn't exist on >=6.4 (probe per-module);
load-nvidia detects foreign nvidia via /sys/module/nvidia/version and
errors with guidance instead of silently skipping.

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