Fix domain name string leak

==00:00:01:17.086 21490== 20 bytes in 1 blocks are definitely lost in loss record 2 of 9
==00:00:01:17.086 21490==    at 0x4C294C4: malloc (vg_replace_malloc.c:292)
==00:00:01:17.086 21490==    by 0x505777A: read_message (xs.c:1145)
==00:00:01:17.086 21490==    by 0x5057AE4: xs_talkv (xs.c:428)
==00:00:01:17.086 21490==    by 0x5057E28: xs_single (xs.c:546)
==00:00:01:17.086 21490==    by 0x5058584: xs_read (xs.c:592)
==00:00:01:17.086 21490==    by 0x5EC15DC: xen_get_name_from_domainid (xen.c:621)
==00:00:01:17.086 21490==    by 0x5EBAB86: vmi_init_private (core.c:437)
==00:00:01:17.086 21490==    by 0x5EBAE8A: vmi_init_custom (core.c:670)