fix: xen not returning absolute block device paths for vdisks

                        elif d_type == 'block':

                            d_path = xdisk.getElementsByTagName('source').pop()\

                                                            .getAttribute('dev')

                            # FIXME sometimes xen do not report '/dev/' at the

                            # beginning of block devices, and relative paths

                            # are non-sense

                            # Example: vg/myvm instead of /dev/vg/myvm

                            if d_path[0] != '/':

                                d_path = '/dev/' + d_path

                        # search the volume object

                        if d_type in ['file', 'block']:

                            volumes.append(self._hv_handle._sto_handle