Hoplug: objects modifications/additions

Introduce new HotplugInfo object as part of Instance object.

hotplug_info { 'nics': 0, 'disks': 0, 'pci_pool': [16, 17, 18] }

nics/disks is an index used for device identification and naming.
Everytime a new device (disk/nic) is hotadded these values get incremented.

pci_pool is a list of pci slots where a device can be placed. Every
time a device gets hot-plugged/unplugged a pci slot gets popped/appended.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>