efi_loader: link partition to block device

We provide a UEFI driver for block devices. When ConnectController() is
called for a handle with the EFI_BLOCK_IO_PROTOCOL this driver creates the
partitions. When DisconnectController() is called the handles for the
partitions have to be deleted. This requires that the child controllers
(partitions) open the EFI_BLOCK_IO_PROTOCOL of the controller (block IO
device) with attribute EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>