migration: add blocktime calculation into migration-test

This patch just requests blocktime calculation,
and check it in case when UFFD_FEATURE_THREAD_ID feature is set
on the host.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>