monitor: do_balloon(): Check for errors

do_balloon() should check for ballooning availability as
do_info_balloon() does.

Noted by Daniel P. Berrange <berrange@redhat.com>.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
