Fix tiny leak in qemu_opts_parse

qemu_opts_create duplicates the id we pass in case it shall be stored in
the opts. So we do not need to dup it in qemu_opts_parse, leaking a few
bytes this way.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>