Add possibility to limit block_copy() call in time. To be used in the
next commit.

As timed-out block_copy() call will continue in background anyway (we
can't immediately cancel IO operation), it's important also give user a
possibility to pass a callback, to do some additional actions on
block-copy call finish.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>