qemu: conf: Don't leak snapshot image format conf variable

qemu: conf: Don't leak snapshot image format conf variable

==20406== 4 bytes in 1 blocks are definitely lost in loss record 6 of 1,059
==20406==    at 0x4C2AF3F: malloc (vg_replace_malloc.c:299)
==20406==    by 0x8F17D39: strdup (in /lib64/libc-2.24.so)
==20406==    by 0x552C0E0: virStrdup (virstring.c:784)
==20406==    by 0x54D3622: virConfGetValueString (virconf.c:945)
==20406==    by 0x144E4692: virQEMUDriverConfigLoadFile (qemu_conf.c:687)
==20406==    by 0x1452A744: qemuStateInitialize (qemu_driver.c:664)
==20406==    by 0x55DB585: virStateInitialize (libvirt.c:770)
==20406==    by 0x124570: daemonRunStateInit (libvirtd.c:881)
==20406==    by 0x5532990: virThreadHelper (virthread.c:206)
==20406==    by 0x8C82493: start_thread (in /lib64/libpthread-2.24.so)
==20406==    by 0x8F7FA1E: clone (in /lib64/libc-2.24.so)