lazy-pages: interleave #PF handling with transfers of remaining pages

Currently we poll userfaultfd for page faults and if there were no page
faults during 5 seconds we stop monitoring the userfaultfd and start
copying remaining pages chunk by chunk.
If a page fault occurs during the copy, the faulting process will be stuck
until the page it accessed would be copied to its address space.
This patch limits the initial "page fault only" stage to 1 second instead
of 5, and interleaves non-blocking poll of userfaultfd with copying of the
remaining memory afterwards.

travis-ci: success for lazy-pages: interleave #PF handling with transfers of remaining pages
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>