core: scheduling: remove migration filter

it seems that without migration filter (filters out the
'migration from' host), the scheduler can select the same source host.
solution: remove the migration filter, and in migrate
vm add the source host to the scheduler host black list.

Change-Id: Ibd1007bbc5be01043203cd4eff1aa52c6fb03a13
Bug-Url: https://bugzilla.redhat.com/1008153
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>