VirtualBox hosts can share folders with guests, this commit adds a
VFS driver implementing the Linux-guest side of this, allowing folders
exported by the host to be mounted under Linux.
This driver depends on the guest host IPC functions exported by
the vboxguest driver.
Acked-by: Christoph Hellwig
Signed-off-by: Hans de Goede
Signed-off-by: Al Viro