drm/xe/vf: Use GUC_HXG_TYPE_EVENT for GuC context register

drm/xe/vf: Use GUC_HXG_TYPE_EVENT for GuC context register

The only case where the GuC submission backend cannot reason 100%
correctly is when a GuC context is registered during VF post-migration
recovery. In this scenario, it's possible that the GuC context register
H2G is processed, but the immediately following schedule-enable H2G gets
lost. The schedule-enable G2H "done" response is how the GuC state machine
determines whether context registration has completed.

A double register is harmless when using `GUC_HXG_TYPE_EVENT`, as GuC
simply drops the duplicate H2G. To keep things simple, use
`GUC_HXG_TYPE_EVENT` for all context registrations on VFs.

v5:
 - Check for xe_sriov_vf_migration_supported (Tomasz)
v7:
 - Add comment about subsequent protocol failures (Tomasz)
 - Modify commit message (Michal)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Tomasz Lis <tomasz.lis@intel.com>
Link: https://lore.kernel.org/r/20251008214532.3442967-20-matthew.brost@intel.com