bhyve: Cast away const when fetching a config nvlist
Silence a warning from the compiler about "const" being discarded. The
warning is correct: nvlist values are supposed to be immutable.
However, fixing this properly will require some contortions on behalf of
consumers who look up a subtree of the config and modify it. Per a
discussion on freebsd-virtualization@, the solution will probably be to
outright replace the use of nvlists for VM configuration, but until that
happens let's document the problem and silence the warning.