qemuAssignDeviceInputAlias: Be tolerant to pre-existent alias

https://bugzilla.redhat.com/show_bug.cgi?id=1554876

This is missing in 6bc4a371 and therefore we are effectively
overwriting user provided aliases for <input/> devices.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_alias.c

@@ -513,6 +513,9 @@ qemuAssignDeviceInputAlias(virDomainDefPtr def,
                            virDomainInputDefPtr input,
                            int idx)
 {
+    if (input->info.alias)
+        return 0;
+
     if (idx == -1) {
         int thisidx;
         size_t i;