core: VM should be PINNED_TO_HOST when using host cpu flags

according to the bug a VM using pass-through host cpu (host cpu flags)
might bump into issues when migrating to cpu lower than the SRC hardware
and this is, apparently not supported

Change-Id: I781891e91680bbe8095097a2a1e4c426e318c720
Bug-Url: https://bugzilla.redhat.com/1052168
Signed-off-by: Roy Golan <rgolan@redhat.com>