migration: fix use-after-free in loadvm_postcopy_handle_run_bh

MigrationState is destroyed before we can come into bottom half.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Amit Shah <amit.shah@redhat.com>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <1457537708-8622-1-git-send-email-den@openvz.org>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
