Fix stale call to deactivateVolumes().

Calling deactivateImage() is correct due to the one volume (no
template limitation of Image._activateVolumeForImportExport().

This calls were introduced by a conflict in changes:
Ib208bb20d88633167b5c66b42febe912439a022d
I4b6a4e99cf3a657affb4f5e96aa1ac1978a297da

Change-Id: I79fa91ddb3f3c41052461ee768157e86442f6660
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17734
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Tested-by: Federico Simoncelli <fsimonce@redhat.com>