vm: wrong message formatting leads to an exception

vm -> diskReplicateStart:
wrong string formatting in error log leads to
the following exception:
"TypeError: not enough arguments for format string".

Change-Id: Ib789adb51cd4ca112489d55b6e10f69698a4a3ac
Signed-off-by: Daniel Erez <derez@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28132
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>