core: block hot unplug CPU

Currently unplug isn't supported due to libvirt's bug[1]

- added canDo action
- exposed config value via engine-confg for
  HotPlugCpuSupported and HotPlugCpuSupported

Change-Id: I32eafdc685198b98005906dc14f6fdc8cc1c7763
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1077515
Signed-off-by: Roy Golan <rgolan@redhat.com>