lazy-pages: update drop_lazy_iovs to handle addresses outside IOVs

Currently drop_lazy_iovs presumes that the range that should be dropped
starts inside an IOV. This works fine with page faults and background pages
but will fail for mapping changes.

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>