vmw_balloon: unify commands tracing and stats

Now that we have a single point, unify the tracing and collecting the
statistics for commands and their failure. While it might somewhat
reduce the control over debugging, it cleans the code a lot.

Reviewed-by: Xavier Deguillard <xdeguillard@vmware.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>