qemuMonitorJSONBlockStatsUpdateCapacity: Don't skip disks

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>