rc.d/iovctl: Start the iovctl service after kld.

Drivers, including those that support SR-IOV, can be loaded via kld_list
and it makes sense to process iovctl configuration files afer all
drivers have been loaded.

MFC after: 1 week
Sponsored by: Chelsio Communications
Reviewed by: 0mp, jhb
Differential Revision: https://reviews.freebsd.org/D47937