qemu: ignore assumptions about hotplug requirement when address is from config

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).