This will allow dynamically adding/removing at least virtio-net pci
devices which are quite the norm in cloud environment with QEMU/KVM

    netdev_add bridge,id=wan2,br=br-wan,helper=/home/yousong/.usr/libexec/qemu-bridge-helper
    device_add virtio-net-pci,id=devwan2,netdev=wan2,mac=11:22:33:22:11:00

The config was formed by selecting target x86/64 first, then select
CONFIG_HOTPLUG_PCI and CONFIG_HOTPLUG_PCI_ACPI with

    make kernel_menuconfig CONFIG_TARGET=subtarget

The following text tries to explain how the current config was formed

 1. CONFIG_PCI_LABEL and CONFIG_ATA_PIIX were removed because they were
    already enabled in x86 platform config
 2. CONFIG_ATA_SFF was removed because it was enabled in generic config
 3. CONFIG_NLS was removed because it will be selected by CONFIG_PCI_LABEL

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>