core, engine: Fix HotPlugCpuSupported config value

The 'HotPlugCpuSupported' configuration value was not present in the
database and another unused property called HotPlugCpuEnable contained
syntax errors that prevented it from being read by the JSON parser.
This patch removes the unused property and adds the correct one, fixing
the syntax errors that were present.

Change-Id: I3ef5b2e39b2067239721a65b031d247ace3b0c2c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1069802
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>