zfsd: Allow zfsd to work on any type of GEOM provider

cddl/usr.sbin/zfsd/zfsd_event.cc
Remove the check for da and ada devices. This way zfsd can work on md,
geli, glabel, gstripe, etc devices. geli in particular is useful
combined with ZFS. gnop is also useful for simulating drive pulls in
the ZFSD test suite.

Also, eliminate the DevfsEvent class entirely. Move its
responsibilities into GeomEvent. We can get everything we need to know
just from listening to GEOM events.

lib/libdevdctl/event.cc
Fix GeomEvent::DevName for CREATE events. Oddly, the relevant field is
named "cdev" for CREATE events but "devname" for disk events.

MFC after: 3 weeks
Relnotes: Yes (probably worth mentioning the geli part)
Sponsored by: Spectra Logic Corp