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