qemu: ignore assumptions about hotplug requirement when address is from config
    
    Certain PCI buses don't support hotplug, and when automatically
    assigning PCI addresses for devices, libvirt is very conservative in
    its assumptions about whether or not a device will need to be
    hotplugged/unplugged in the future. But if the user manually assigns
    an address, they likely are aware of any hotplug requirements of the
    device (or at least they should be).
    
    In short, after this patch, automatically PCI address assignment will
    assume that the device must be plugged in to a hot-pluggable slot, but
    manually assignment can place the device in any bus that is
    compatible, regardless of whether or not it supports hotplug. If the
    user makes a mistake and plugs the device into a bus that doesn't
    support hotplug, then later tries to do a hot-unplug, qemu will give
    an appropriate error.
    
    (in the future we may want to add a "hotpluggable" attribute to all
    devices, with default being "yes" for autoassign, and "no" for manual
    assign).