Update error message for multifd tunnelled_migration

The error message has been updated recently, so modify it
accordingly.
Signed-off-by: NYingshun Cui <yicui@redhat.com>

libvirt/tests/cfg/migration/migrate_options_shared.cfg

@@ -557,6 +557,5 @@
                     variants:
                         - multifd:
                             virsh_migrate_extra = "--parallel"
-                            # TODO: Need to update error message once BZ 1726937 got fixed
-                            err_msg = "unsupport"
+                            err_msg = "Flags 'VIR_MIGRATE_TUNNELLED' and 'VIR_MIGRATE_PARALLEL' are mutually exclusive"