frontend: disable cores-per-socket for running VM

This patch makes the cores per virtual socket list unchangeable for a
running VM, just as the total virtual CPUs field and the virtual sockets
list are unchangeable for running VM.

Change-Id: I552114e2ef7f81c920b091ca19c3811ecf36b2ad
Bug-Url: https://bugzilla.redhat.com/922609
Signed-off-by: Arik Hadas <ahadas@redhat.com>