lazy-pages: add handling of UFFD_EVENT_REMOVE

When the restored process calls madvise(MADV_DONTNEED) or
madvise(MADV_REMOVE) the memory range specified by the madvise() call
should be remapped to zero pfn and we should stop monitoring this range in
order to avoid its pollution with data the process does not expect.
All we need to do here, is to unregister the memory range from userfaultfd
and the kernel will take care of the rest.

travis-ci: success for lazy-pages: add non-#PF events handling (rev2)
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>