sandbox: pci: Remember the device being emulated

sandbox: pci: Remember the device being emulated

Add a field to the PCI emulator per-device data which records which device
is being emulated. This is useful when the emulator needs to check the
device for something.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: rebase the patch against u-boot-x86/master to get it applied cleanly]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>