drm/i915/gvt: Change KVMGT as self load module

This trys to make 'kvmgt' module as self loadable instead of loading
by i915/gvt device model. So hypervisor specific module could be
stand-alone, e.g only after loading hypervisor specific module, GVT
feature could be enabled via specific hypervisor interface, e.g VFIO/mdev.

So this trys to use hypervisor module register/unregister interface
for that. Hypervisor module needs to take care of module reference
itself when working for hypervisor interface, e.g for VFIO/mdev,
hypervisor module would reference counting mdev when open and release.

This makes 'kvmgt' module really split from GVT device model. User
needs to load 'kvmgt' to enable VFIO/mdev interface.

v6:
- remove unused variable

v5:
- put module reference in register error path

v4:
- fix checkpatch warning

v3:
- Fix module reference handling for device open and release. Unused
  mdev devices would be cleaned up in device unregister when module unload.

v2:
- Fix kvmgt order after i915 for built-in case

Cc: "Yuan, Hang" <hang.yuan@intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: "He, Min" <min.he@intel.com>
Reviewed-by: Yuan, Hang <hang.yuan@intel.com>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/hypercall.h

@@ -33,11 +33,17 @@
 #ifndef _GVT_HYPERCALL_H_
 #define _GVT_HYPERCALL_H_
 
+enum hypervisor_type {
+	INTEL_GVT_HYPERVISOR_XEN = 0,
+	INTEL_GVT_HYPERVISOR_KVM,
+};
+
 /*
  * Specific GVT-g MPT modules function collections. Currently GVT-g supports
  * both Xen and KVM by providing dedicated hypervisor-related MPT modules.
  */
 struct intel_gvt_mpt {
+	enum hypervisor_type type;
 	int (*host_init)(struct device *dev, void *gvt, const void *ops);
 	void (*host_exit)(struct device *dev);
 	int (*attach_vgpu)(void *vgpu, unsigned long *handle);
@@ -67,6 +73,5 @@ struct intel_gvt_mpt {
 };
 
 extern struct intel_gvt_mpt xengt_mpt;
-extern struct intel_gvt_mpt kvmgt_mpt;
 
 #endif /* _GVT_HYPERCALL_H_ */