Acquire the yield mutex in the VirtualDevice destructor only in a dbgutil
build on OS X. (I added that acquire yesterday to fix an assertion when
exiting on OS X.) Seems to fix a hang in "make subsequentcheck" on Linux. No
idea what is going on exactly, sigh.

Change-Id: Ifcb1897720998d0725a8db0d5f8c75848c6a30f7