Raise an exception for kernels that do not have userfaultfd. For the
kernels that have userfaultfd but do not provide non-cooperative events
(4.3 - 4.11) just print a warning.

Fixes: #363
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>