qemuMonitorJSONBlockStatsUpdateCapacity: Don't skip disks
    
    The function queries the block devices visible to qemu
    ('query-block') and parses the qemu's output. The info is
    returned in a hash table which is expected to be pre-filled by
    qemuMonitorJSONGetAllBlockStatsInfo(). However, in the next patch
    we are not going to call the latter function at all, so we should
    make the former function add devices into the hash table if not
    found there.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>