vfio/pci: Expose latched module parameter policy in debugfs

The nointxmask and disable_idle_d3 module parameters remain writable,
but vfio-pci now latches their values into each device at init.  Once a
device is registered, changing the module parameter only affects future
devices, leaving no direct way to confirm the effective policy for an
existing device.

Add a pci debugfs directory under the VFIO device debugfs root and
report the per-device nointxmask and disable_idle_d3 values.  These are
read-only debugfs views and use the same Y/N bool output convention as
the module parameters.

Read-only vfio-pci parameters, such as disable_vga, are not exposed here
because they cannot drift from the latched device value, therefore the
existing module parameter exposure via sysfs is sufficient.

Note that while only vfio-pci currently provides these options, the
implementation is in vfio-pci-core and therefore properly reflects the
device policy in the core, regardless of driver.

Assisted-by: OpenAI Codex:gpt-5
Cc: Guixin Liu <kanie@linux.alibaba.com>
Signed-off-by: Alex Williamson <alex.williamson@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20260615191241.688297-7-alex.williamson@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

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