vl.c: fix memory leak spotted by valgrind

valgrind complains about:
==42062== 16 bytes in 1 blocks are definitely lost in loss record 387 of 1,048
==42062==    at 0x402DCB2: malloc (vg_replace_malloc.c:299)
==42062==    by 0x40C1BE3: g_malloc (in /usr/lib64/libglib-2.0.so.0.3800.2)
==42062==    by 0x40DA133: g_slice_alloc (in /usr/lib64/libglib-2.0.so.0.3800.2)
==42062==    by 0x40DB2E5: g_slist_prepend (in /usr/lib64/libglib-2.0.so.0.3800.2)
==42062==    by 0x801637FF: object_class_get_list_tramp (object.c:690)
==42062==    by 0x40A96C9: g_hash_table_foreach (in /usr/lib64/libglib-2.0.so.0.3800.2)
==42062==    by 0x80164885: object_class_foreach (object.c:665)
==42062==    by 0x80164975: object_class_get_list (object.c:698)
==42062==    by 0x800100A5: machine_parse (vl.c:2447)
==42062==    by 0x800100A5: main (vl.c:3756)

Lets free machines in case of mc.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
