libxl: fix memory leak in libxl_name_to_domid

libxl: fix memory leak in libxl_name_to_domid

Found with "valgrind xl destroy ...":
==16272== 53,248 bytes in 1 blocks are definitely lost in loss record 6 of 6
==16272==    at 0x4022249: calloc (vg_replace_malloc.c:467)
==16272==    by 0x403FD4A: libxl_list_domain (libxl.c:490)
==16272==    by 0x404B901: libxl_name_to_domid (libxl_utils.c:65)
==16272==    by 0x804B4D2: domain_qualifier_to_domid (xl_cmdimpl.c:181)
==16272==    by 0x804B50F: find_domain (xl_cmdimpl.c:198)
==16272==    by 0x804D70C: destroy_domain (xl_cmdimpl.c:2104)
==16272==    by 0x8054E4C: main_destroy (xl_cmdimpl.c:2912)
==16272==    by 0x804B2FB: main (xl.c:76)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>