qemu: hotplug: Don't leak 'nodename' in qemuDomainChangeMediaBlockdev

qemuDomainDiskGetBackendAlias allocates a copy of the nodename string so
we need to free it at the end.

Found by Coverity.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_hotplug.c

@@ -700,6 +700,7 @@ qemuDomainChangeMediaBlockdev(virQEMUDriverPtr driver,
  cleanup:
     qemuHotplugDiskSourceDataFree(newbackend);
     qemuHotplugDiskSourceDataFree(oldbackend);
+    VIR_FREE(nodename);
     /* caller handles correct exchange of sources */
     disk->src = oldsrc;
     return ret;