conf: introduce option to enable/disable pci hotplug on pci-root controller

conf: introduce option to enable/disable pci hotplug on pci-root controller

This change introduces libvirt xml support to enable/disable hotplug on the
pci-root controller. It adds a 'target' subelement for the pci-root controller
with a 'hotplug' property. This property can be used to enable or disable
hotplug for the pci-root controller. For example, in order to disable hotplug
on the pci-root controller, one has to use set '<target hotplug='off'>' as
shown below:

<controller type='pci' model='pci-root'>
  <target hotplug='off'/>
</controller>

'<target hotplug='on'>' option would enable hotplug for pci-root controller.
This is also the default value. This option is only available for pc machine
types and is applicable for qemu/kvm accelerator only.This feature was
introduced from qemu version 5.2 with the following change in qemu repository:

3d7e78aa7777f ("Introduce a new flag for i440fx to disable PCI hotplug on the root bus")

The above qemu commit describes some reasons why users might to disable hotplug
on PCI root buses.

Related unit tests to exercise the new conf option has also been added.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>