This stub was previously using libossetup_subnode as its base class,
but the constructor was calling the base class constructor with a
"type" parameter, where the base class did not have a corresponding
argument.  This commit changes the stub class for blockstores to
refer directly to libossetup_handler instead.  We can sort out
whether going through some sort of "libossetup_subnode" base class
makes sense a bit later.
(cherry picked from commit 89fb668999deb76913957a8abd8950f8b60eccaf)