lazy-pages: simplify background transfer logic

First check if there are pages we need to transfer and only afterwards
check if there are outstanding requests. Also, instead checking 'bool
remaining' to see if there is more work to do we can simply check if all
the lpi's have been already serviced.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>