xl: Support (by ignoring) xl migrate --live

xm migrate would do non-live migration (effectively, save, transfer
and restore) by default, unless you specified --live.

xl migrate always does live migration.  Honour (by ignoring) --live
for compatibility with old callers.  Document this.

(This patch should be backported as far as possible.)

Reported-by: Matthew Vernon <mcv21@cam.ac.uk>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Matthew Vernon <mcv21@cam.ac.uk>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

docs/man/xl.pod.1

@@ -1520,6 +1520,14 @@ monitor types are:
 
 =back
 
+=head1 IGNORED FOR COMPATIBILITY WITH XM
+
+xl is mostly command-line compatible with the old xm utility used with
+the old Python xend.  For compatibility, the following options are
+ignored:
+
+=item B<xl migrate --live>
+
 =head1 TO BE DOCUMENTED
 
 We need better documentation for:

tools/libxl/xl_cmdimpl.c

@@ -4521,6 +4521,7 @@ int main_migrate(int argc, char **argv)
     int opt, daemonize = 1, monitor = 1, debug = 0;
     static struct option opts[] = {
         {"debug", 0, 0, 0x100},
+        {"live", 0, 0, 0x200},
         COMMON_LONG_OPTS,
         {0, 0, 0, 0}
     };
@@ -4539,9 +4540,12 @@ int main_migrate(int argc, char **argv)
         daemonize = 0;
         monitor = 0;
         break;
-    case 0x100:
+    case 0x100: /* --debug */
         debug = 1;
         break;
+    case 0x200: /* --live */
+        /* ignored for compatibility with xm */
+        break;
     }
 
     domid = find_domain(argv[optind]);