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.

Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D39437

(cherry picked from commit 92642bba4db4bf1719758ac7233468bc09a0bd59)

Fix zfsd unittests after 92642bba4db

At the time we ensured that the more important functional tests were
working, but neglected to update the unit tests.

Sponsored by: Axcient

(cherry picked from commit 2a0c0aea42092f89c2a5345991e6e3ce4cbef99a)