vm: Fix exception when shutdown() called on vm in 'Down' state

Attempting to call the shutdown() method on a VM which is in 'Down'
state resulted in an exception if called through xmlrpclib.  Instead,
adopt the convention used by similar APIs such as migrate() and return
errCode['noVM'].

Change-Id: I8b75d0b869354d503e5f187e33370b5837b3c698
Bug-Url: https://bugzilla.redhat.com/1027297
Signed-off-by: Greg Padgett <gpadgett@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21937
Reviewed-by: Sandro Bonazzola <sbonazzo@redhat.com>
Reviewed-by: Doron Fediuck <dfediuck@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>