core: Avoid starting vm failed if pin to host is removed

If the vm configured vm numa nodes and pinned them to host numa nodes,
when the host was removed,the pinned host numa nodes would be removed
also. Now if you try to run the vm, it would fail. This patch will
avoid this situation.

Change-Id: Icbbfd981d0996689cdc818bdd02e31155799d55c
Bug-Url: https://bugzilla.redhat.com/1101387
Signed-off-by: Bruce Shi <xiao-lei.shi@hp.com>