qemu_driver: Resolve Coverity RESOURCE_LEAK

Coverity found that the 'buf' wasn't VIR_FREE'd at exit.

src/qemu/qemu_driver.c

@@ -10516,6 +10516,7 @@ qemuDomainGetBlockInfo(virDomainPtr dom,
     }
 
  cleanup:
+    VIR_FREE(buf);
     VIR_FREE(alias);
     virStorageSourceFree(meta);
     VIR_FORCE_CLOSE(fd);