migration: fix small leaks

Spotted thanks to valgrind and tests/device-introspect-test:

==11711== 1 bytes in 1 blocks are definitely lost in loss record 6 of 14,537
==11711==    at 0x4C2EB6B: malloc (vg_replace_malloc.c:299)
==11711==    by 0x1E0CDBD8: g_malloc (gmem.c:94)
==11711==    by 0x1E0E696E: g_strdup (gstrfuncs.c:363)
==11711==    by 0x695693: migration_instance_init (migration.c:2226)
==11711==    by 0x717C4B: object_init_with_type (object.c:344)
==11711==    by 0x717E80: object_initialize_with_type (object.c:375)
==11711==    by 0x7182EB: object_new_with_type (object.c:483)
==11711==    by 0x718328: object_new (object.c:493)
==11711==    by 0x4B8A29: qmp_device_list_properties (qmp.c:542)
==11711==    by 0x4A9561: qmp_marshal_device_list_properties (qmp-marshal.c:1425)
==11711==    by 0x819D4A: do_qmp_dispatch (qmp-dispatch.c:104)
==11711==    by 0x819E82: qmp_dispatch (qmp-dispatch.c:131)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170801160419.14180-1-marcandre.lureau@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
