Fix vtnet hang with max_virtqueue_pairs > VTNET_MAX_QUEUE_PAIRS

Correctly limit npairs passed to vtnet_ctrl_mq_cmd. This ensures that
VQ_ALLOC_INFO_INIT is called with the correct value, preventing the system
from hanging when max_virtqueue_pairs > VTNET_MAX_QUEUE_PAIRS.

Add new sysctl requested_vq_pairs which allow the user to configure
the requested number of virtqueue pairs. The actual value will still take
into account the system limits.

Also missing sysctls for the current tunables so their values can be seen.

PR: 207446
Reported by: Andy Carrel
MFC after: 3 days
Relnotes: Yes
Sponsored by: Multiplay