Kernel/Devices: Introduce the LoopDevice device

This device is a block device that allows a user to effectively treat an
Inode as a block device.

The static construction method is given an OpenFileDescription reference
but validates that:
- The description has a valid custody (so it's not some arbitrary file).
  Failing this requirement will yield EINVAL.
- The description custody points to an Inode which is a regular file, as
  we only support (seekable) regular files. Failing this requirement
  will yield ENOTSUP.

LoopDevice can be used to mount a regular file on the filesystem like
other supported types of (physical) block devices.

Kernel/FileSystem/FileSystem.h

@@ -34,6 +34,11 @@ public:
     virtual Inode& root_inode() = 0;
     virtual bool supports_watchers() const { return false; }
 
+    // FIXME: We should aim to provide more concise mechanism to ensure
+    // that backing Inodes from the FileSystem are kept intact so we can
+    // attach them to a loop device.
+    virtual bool supports_backing_loop_devices() const { return false; }
+
     bool is_readonly() const { return m_readonly; }
 
     virtual unsigned total_block_count() const { return 0; }