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
3dbd001cc5