rust: callbacks: allow passing optional callbacks as ()

rust: callbacks: allow passing optional callbacks as ()

In some cases, callbacks are optional.  Using "Some(function)" and "None"
does not work well, because when someone writes "None" the compiler does
not know what to use for "F" in "Option<F>".

Therefore, adopt () to mean a "null" callback.  It is possible to enforce
that a callback is valid by adding a "let _: () = F::ASSERT_IS_SOME" before
the invocation of F::call.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>