a <controller type='pci'...> element can now have a "hotplug"
attribute in the <target> subelement. This is intended to control
whether or not the slot(s) of the controller support
hotplugging/unplugging a device:

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

The default value of hotplug is "on".

Since support for configuring such an option is hypervisor-dependent
(and will vary among different types of PCI controllers even on a
single hypervisor), no validation is done in this patch - that
validation will be done in the patch that wires support for the
setting into the hypervisor.
Signed-off-by: NLaine Stump <laine@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>