core: reset migration progress on migration end

There where 2 problems:
- if the qemu process died on source host the migration progress was never
  nulled
- if the qemu process died on the destination host the migration progress was
  never nulled

Change-Id: Ifeb3875da032622d53f3921b0186b08009e4a191
Bug-Url: https://bugzilla.redhat.com/1114997
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>