qemuBuildHostNetStr: Explicitly enumerate net types

We tend to prevent using 'default' in switches. And it is for a
good reason - control may end up in paths we wouldn't want for
new values. In this specific case, if qemuBuildHostNetStr is
called over VIR_DOMAIN_NET_TYPE_VHOSTUSER it would produce
meaningless output. Fortunately, there no such call yet.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_command.c

@@ -3723,9 +3723,21 @@ qemuBuildHostNetStr(virDomainNetDefPtr net,
         break;
 
     case VIR_DOMAIN_NET_TYPE_USER:
-    default:
+    case VIR_DOMAIN_NET_TYPE_INTERNAL:
         virBufferAddLit(&buf, "user");
         break;
+
+    case VIR_DOMAIN_NET_TYPE_HOSTDEV:
+        /* Should have been handled earlier via PCI/USB hotplug code. */
+        virObjectUnref(cfg);
+        return NULL;
+
+    case VIR_DOMAIN_NET_TYPE_VHOSTUSER:
+        /* Unsupported yet. */
+        break;
+
+    case VIR_DOMAIN_NET_TYPE_LAST:
+        break;
     }
 
     if (vlan >= 0) {