pci: don't abort() when trying to hotplug with acpi off.

The PCI bus on x86 requires ACPI for hotplug support, thus disbling ACPI
also disables hotplug for the PCI bus.  This patch makes qemu check
whenever the PCI bus in question can handle hotplug before trying to add
devices.  This is needed because qdev will abort() on any attempt to
hotplug devices into a non-hotpluggable bus.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
