vfs: pass root directory to plugins

This patch enables the use of the VFS from VFS plugins by passing a
reference of the root directory to the constructors of file-system
instances. Since it changes the signature of 'Vfs::Dir_file_system',
any code that uses the VFS directly requires an adaptation.

Fixes #2701

repos/dde_linux/src/lib/vfs/lxip/vfs.cc

@@ -1718,7 +1718,8 @@ struct Lxip_factory : Vfs::File_system_factory
 	Vfs::File_system *create(Genode::Env       &env,
 	                         Genode::Allocator &alloc,
 	                         Genode::Xml_node  config,
-	                         Vfs::Io_response_handler &io_handler) override
+	                         Vfs::Io_response_handler &io_handler,
+	                         Vfs::File_system &) override
 	{
 		static Init inst(env, alloc);
 		return new (alloc) Vfs::Lxip_file_system(env, alloc, config, io_handler);