monitor: refactor error_vprintf()

The monitor_vprintf() code will return -1 if either the monitor
is NULL, or the monitor is QMP. The error_vprintf() code can
take advantage of this to avoid having to duplicate the same
checks, and instead simply look at the return value.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>