drm/xe/pf: Allow to stop the VF using sysfs

drm/xe/pf: Allow to stop the VF using sysfs

It is expected that VFs activity will be monitored and in some
cases admin might want to silence specific VF without killing
the VM where it was attached.

Add write-only attribute to stop GuC scheduling at VFs level.

  /sys/bus/pci/drivers/xe/BDF/
  ├── sriov_admin/
      ├── vf1/
      │   └── stop		[WO] bool
      ├── vf2/
      │   └── stop		[WO] bool

Writing "1" or "y" (or whatever is recognized by the strtobool()
function) to this file will trigger the change of the VF state
to STOP (GuC will stop servicing the VF). To go back to a READY
state (to allow GuC to service this VF again) the VF FLR must be
triggered (which can be done by writing 1 to device/reset file).

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patch.msgid.link/20251030222348.186658-17-michal.wajdeczko@intel.com