efi_loader: limit output length for VenHw, VenMedia

VenHw and VenMedia device path nodes may carry vendor defined data of
arbitrary length. When converting a device path node to text ensure that we
do not overrun our internal buffer.

In our implementation of
EFI_DEVICE_PATH_TO_TEXT_PROTOCOL.ConvertDevicePathToText() we could first
determine the output length and then allocate buffers but that would nearly
double the code size. Therefore keep the preallocated buffers and truncate
excessive device paths instead.

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