conf: during PCI hotplug, require that the controller support hotplug

Before this patch we would simply rely on QEMU failing to attach the
device. Since we have a flag in the address set telling us which
controllers support hotplug, we can fail the operation sooner.

This also assures that when hotplugging with no provided PCI address,
that we skip any controllers with hotplug='off', and attempt to assign
the device to a controller that not only supports hotplug, but also
has it enabled.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>