drm/i915/gvt/kvmgt: mdev ABI is available_instances, not available_instance

Per the ABI specification[1], each mdev_supported_types entry should
have an available_instances, with an "s", not available_instance.

[1] Documentation/ABI/testing/sysfs-bus-vfio-mdev

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>