[fast-reboot] Reduce time for QosOrch by not query 7x queues

[kcudnik]

time should be reduced from 6 sec to about 1.5 sec

Please refer here sonic-net/sonic-buildimage#8428

[stephenxs]

Just FYI.
The QoS orchagent calls SAI APIs to get the number of child scheduler groups and then initialize them.
After that, the size of child scheduler groups vector will be non-zero, which indicates the child scheduler groups have been initialized and prevent QoS orchagent from calling SAI APIs.
However, on Mellanox platform, some scheduler groups don't have child group, leaving size of child scheduler groups always being zero.
This causes QoS orchagent to call the SAI API each time the scheduler group is handled.
This can be fixed by introducing another flag indicating whether the child group size has been initialized.

[stephenxs]

Fixed by #1965

[kcudnik]

Closing since fixed