vdsmd.init: Add service-is-managed in shutdown_conflicting_srv

We should not throw a stack trace if a service does not exist

Change-Id: I8916a73c446ea2f296b66f5fcf133b07c2d7a66c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1006842
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19197
Reviewed-by: Dan Kenigsberg <danken@redhat.com>