spapr_event: changed hotplug event type from slot to pci

The hotplug event only defines slot and phb types. Current hotplug implementation
performs physical style hotplug as opposed to the logical (slot) hotplug. Added
new hotplug type to surface to the guest to ensure that drmgr is run wiht "-c pci".

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>