lazy-pages: prepare for non-#PF events handling

Rename handle_user_fault to handle_uffd_event and split actual page fault
handling to a helper function

travis-ci: success for lazy-pages: add non-#PF events handling
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>