When the restored process calls mremap(), we will see #PF's on the new
addresses and we have to create a correspondence between the addresses
found in the dump and the actual addresses the process uses. For this
purpose we distinguish "live" address and "image" address in the lazy IOVs
and outstanding requests. The "live" address is used to find the
appropriate IOV and in uffd_copy and the "image" address is used to request
pages from the page-read.

If the mremap() call causes the mapping to grow, the additional part will
receive zero pages, as expected.

For the shrinking remaps, we will get UFFD_EVENT_UNMAP for the dropped part
of the mapping.

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>