hw/sd/sdhci.c: Remove x-drive property

hw/sd/sdhci.c: Remove x-drive property

The following commits will remove support for the old sdhci-pci
command line syntax using the x-drive property:
 -device sdhci-pci,x-drive=mydrive -drive id=mydrive,[...]
and replace it with an explicit sd device:
 -device sdhci-pci -drive id=mydrive,[...] -device sd,drive=mydrive

(This is OK because x-drive is experimental.)

This commit removes the x-drive property so that old style
command lines will fail with a reasonable error message:
  -device sdhci-pci,x-drive=mydrive: Property '.x-drive' not found

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1455646193-13238-2-git-send-email-peter.maydell@linaro.org