BindingJsonRpc: correctly stop reactors

vdsm raises an exception saying "AttributeError: 'str' object has no
attribute 'stop'". This commit fixes this problem.

Change-Id: I1060d04bb37b8732eee1678f40cb770716b0473e
Signed-off-by: huntxu <mhuntxu@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/16473
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>