MFC r305013:

Add an empty virtual destructor to zfsd's Vdev class. This is needed
because the class has virtual functions, and the compiler-generated
default destructor is non-virtual.

Reviewed by: asomers

MFC r305016:

Fix the zfsd unittest:

• TESTSDIR is supposed to be under cddl/usr.sbin, not cddl/sbin
• DevdCtl::EventBuffer no longer exists, so remove its forward declaration