qapi: Fix memory leak

valgrind report:

==24534== 232 bytes in 2 blocks are definitely lost in loss record 1,245 of 1,601
==24534==    at 0x4824F20: malloc (vg_replace_malloc.c:236)
==24534==    by 0x293C88: malloc_and_trace (vl.c:2281)
==24534==    by 0x489AD99: ??? (in /lib/libglib-2.0.so.0.2400.1)
==24534==    by 0x489B23B: g_malloc0 (in /lib/libglib-2.0.so.0.2400.1)
==24534==    by 0x2B4EFC: opts_visitor_new (opts-visitor.c:376)
==24534==    by 0x29DEA5: net_client_init (net.c:708)
==24534==    by 0x29E6C7: net_init_client (net.c:966)
==24534==    by 0x2C2179: qemu_opts_foreach (qemu-option.c:1114)
==24534==    by 0x29E85B: net_init_clients (net.c:1008)
==24534==    by 0x296F40: main (vl.c:3463)

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
