pci: sandbox: emul: Rename priv structure

We have "struct sandbox_pci_priv" in pci_sandbox driver. To avoid
confusion, rename the emul's priv to "struct sandbox_pci_emul_priv".

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>