The number of queues that should be return by the admin command should:
1) Only mention the number of non-admin queues.
2) It is zero-based, meaning that '0 == one non-admin queue',
'1 == two non-admin queues', and so forth.
Because our `num_queues` means the number of queues _plus_ the admin
queue, then the right calculation for the number returned from the admin
command is `num_queues - 2`, combining the two requirements mentioned.
The issue was discovered by reducing num_queues from 64 to 8 and running
a Linux VM with an SMP parameter larger than that (e.g. 22). It tries to
utilize all queues, and therefore fails with an invalid queue number
when trying to queue I/Os on the last queue.