core: prevent maintenance a host with hard affinity

Maintenance a host can break the affinity rule of its running vm
The user should be aware of it, and manually handle it:
soften the affinity rule or migrate the VMs.

Change-Id: Idd93ddca562bf739b2cd423e907be1b6da861735
Bug-Url: https://bugzilla.redhat.com/1084794
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>