nodeinfo: fix nodeGetFreePages when max node is zero
    
    In nodeGetFreePages, if startCell is given by '0',
    and the max node number is '0' too. The for-loop
    wouldn't be executed.
    So convert it to while-loop.
    
    Before:
    > virsh freepages --cellno 0 --pagesize 4
    error: internal error: no suitable info found
    
    After:
    > virsh freepages --cellno 0 --pagesize 4
    4KiB: 472637
    
    Signed-off-by: Jincheng Miao <jmiao@redhat.com>
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>