kernel: Move get_mount_point from VMUtils to KPartition.

There's nothing VM-specific about it; it just builds paths
that partitions should be mounted at.

Also move the tests.

headers/private/kernel/disk_device_manager/KPartition.h

@@ -115,15 +115,14 @@ public:
 	virtual status_t GetPath(KPath *path) const;
 		// no setter (see BDiskDevice) -- built on the fly
 
+	status_t GetMountPoint(KPath* mountPoint) const;
+
 	void SetVolumeID(dev_t volumeID);
 	dev_t VolumeID() const;
 
 	void SetMountCookie(void *cookie);
 	void *MountCookie() const;
 
-	virtual status_t Mount(uint32 mountFlags, const char *parameters);
-	virtual status_t Unmount();
-
 	// Parameters
 
 	status_t SetParameters(const char *parameters);