spapr_pci: add dynamic-reconfiguration option for spapr-pci-host-bridge

spapr_pci: add dynamic-reconfiguration option for spapr-pci-host-bridge

This option enables/disables PCI hotplug for a particular PHB.

Also add machine compatibility code to disable it by default for machine
types prior to pseries-2.4.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[agraf: move commas for compat fields]
Signed-off-by: Alexander Graf <agraf@suse.de>