efi_loader: check device path is not installed twice

Prior to corrective patches for virtio and SATA devices the same device
path was installed on two different handles. This is not allowable.
With this patch we will throw an error if this condition occurs for
block devices.

Update a comment for the installation of the simple file system
protocol.

Reported-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>