migration: always use vm_stop_force_state

migration: always use vm_stop_force_state

vm_stop_force_state does:

    if (runstate_is_running()) {
        vm_stop(state);
    } else {
        runstate_set(state);
    }

migration.c does:

    if (runstate_is_running()) {
        vm_stop(state);
    } else {
        vm_stop_force_state(state);
    }

The code run is the same even if we always use vm_stop_force_state in
migration.c.

Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>