xenconfig: add default in xenParseXLDisk()'s switches

Without a default: case in the switches in xenParseXLDisk(), build
would fail with every new disk backend or image format added in libxl,
as this is the case in this error:

http://logs.test-lab.xenproject.org/osstest/logs/103325/build-amd64-libvirt/5.ts-libvirt-build.log

src/xenconfig/xen_xl.c

@@ -393,6 +393,12 @@ xenParseXLDisk(virConfPtr conf, virDomainDefPtr def)
 
                 case LIBXL_DISK_FORMAT_EMPTY:
                     break;
+
+                default:
+                    virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                                   _("disk image format not supported: %s"),
+                                   libxl_disk_format_to_string(libxldisk->format));
+                    goto fail;
                 }
 
                 switch (libxldisk->backend) {
@@ -415,6 +421,11 @@ xenParseXLDisk(virConfPtr conf, virDomainDefPtr def)
                         goto fail;
                     virDomainDiskSetType(disk, VIR_STORAGE_TYPE_BLOCK);
                     break;
+                default:
+                    virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                                   _("disk backend not supported: %s"),
+                                   libxl_disk_backend_to_string(libxldisk->backend));
+                    goto fail;
                 }
             }