[devmgr][memfs] enable pipeline-style connections to the root fs

Change-Id: I5eebb05ef28b77823826d868b50870f918f99395

system/core/devmgr/memfs-private.h

@@ -174,7 +174,9 @@ void vfs_global_init(VnodeDir* root);
 
 // generate mxremoteio handles
 mx_handle_t vfs_create_global_root_handle(void);
+mx_status_t vfs_connect_global_root_handle(mx_handle_t h);
 mx_handle_t vfs_create_root_handle(VnodeMemfs* vn);
+mx_status_t vfs_connect_root_handle(VnodeMemfs* vn, mx_handle_t h);
 
 // device fs
 mx_status_t devfs_mount(mx_handle_t h);

system/core/devmgr/vfs-memory.cpp

@@ -809,3 +809,8 @@ mx_handle_t vfs_create_root_handle(VnodeMemfs* vn) {
     }
     return h2.release();
 }
+
+mx_status_t vfs_connect_root_handle(VnodeMemfs* vn, mx_handle_t h) {
+    mx::channel ch(h);
+    return memfs::vfs.ServeDirectory(fbl::RefPtr<fs::Vnode>(vn), fbl::move(ch));
+}

system/core/devmgr/vfs-rpc.cpp

@@ -51,3 +51,7 @@ void vfs_global_init(VnodeDir* root) {
 mx_handle_t vfs_create_global_root_handle() {
     return vfs_create_root_handle(memfs::global_vfs_root);
 }
+
+mx_status_t vfs_connect_global_root_handle(mx_handle_t h) {
+    return vfs_connect_root_handle(memfs::global_vfs_root, h);
+}