nvmf: Switch several ioctls to using nvlists

For requests that handoff queues from userspace to the kernel as well
as the request to fetch reconnect parameters from the kernel, switch
from using flat structures to nvlists. In particular, this will
permit adding support for additional transports in the future without
breaking the ABI of the structures.

Note that this is an ABI break for the ioctls used by nvmf(4) and
nvmft(4). Since this is only present in main I did not bother
implementing compatability shims.

Inspired by: imp (suggestion on a different review)
Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D48230