virQEMUDriverGetConfig: Fix memleak

virQEMUDriverGetConfig: Fix memleak

==19015== 968 (416 direct, 552 indirect) bytes in 1 blocks are definitely lost in loss record 999 of 1,049
==19015==    at 0x4C2C070: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==19015==    by 0x52ADF14: virAllocVar (viralloc.c:560)
==19015==    by 0x5302FD1: virObjectNew (virobject.c:193)
==19015==    by 0x1DD9401E: virQEMUDriverConfigNew (qemu_conf.c:164)
==19015==    by 0x1DDDF65D: qemuStateInitialize (qemu_driver.c:666)
==19015==    by 0x53E0823: virStateInitialize (libvirt.c:777)
==19015==    by 0x11E067: daemonRunStateInit (libvirtd.c:905)
==19015==    by 0x53201AD: virThreadHelper (virthread.c:206)
==19015==    by 0xA1EE1F2: start_thread (in /lib64/libpthread-2.19.so)
==19015==    by 0xA4EFC8C: clone (in /lib64/libc-2.19.so)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>