drm/i915/gvt: Get reference to KVM iff attachment to VM is successful

Get a reference to KVM if and only if a vGPU is successfully attached to
the VM to avoid leaking a reference if there's no available vGPU.  On
open_device() failure, vfio_device_open() doesn't invoke close_device().

Fixes: 421cfe65 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
Cc: stable@vger.kernel.org
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20221111002225.2418386-2-seanjc@google.com

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -664,8 +664,6 @@ static int intel_vgpu_open_device(struct vfio_device *vfio_dev)
 		return -ESRCH;
 	}
 
-	kvm_get_kvm(vgpu->vfio_device.kvm);
-
 	if (__kvmgt_vgpu_exist(vgpu))
 		return -EEXIST;
 
@@ -676,6 +674,7 @@ static int intel_vgpu_open_device(struct vfio_device *vfio_dev)
 
 	vgpu->track_node.track_write = kvmgt_page_track_write;
 	vgpu->track_node.track_flush_slot = kvmgt_page_track_flush_slot;
+	kvm_get_kvm(vgpu->vfio_device.kvm);
 	kvm_page_track_register_notifier(vgpu->vfio_device.kvm,
 					 &vgpu->track_node);