Update the SDHCI code to use the new SDBus APIs.

This commit introduces the new command line options required
to connect a disk to sdhci-pci:

 -device sdhci-pci -drive id=mydrive,[...] -device sd,drive=mydrive

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1455646193-13238-6-git-send-email-peter.maydell@linaro.org