zfsd: listen for sysevent.fs.zfs instead of misc.fs.zfs

At some point the names of these devd events changed. Probably it
happened when importing OpenZFS. Before that, FreeBSD's sysevent_alloc
method didn't create a "class" nvpair in the event, which led to
log_sysevent using the event's ev_subclass field as its type.

MFC after: 2 weeks
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D39437