libxl: print message how to recover from xl cpupool-cpu-remove errors

An error occurring when calling "xl cpupool-cpu-remove" might leave
the system in a state where a cpu is neither completely free nor in
a cpupool. This can easily be repaired by adding the cpu via
"xl cpupool-cpu-add" to the cpupool where it was removed from before.
Print a message telling this the user in case of an error.

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/xl_cmdimpl.c

@@ -8011,8 +8011,10 @@ int main_cpupoolcpuremove(int argc, char **argv)
         goto out;
     }
 
-    if (libxl_cpupool_cpuremove_cpumap(ctx, poolid, &cpumap))
-        fprintf(stderr, "some cpus may not have been removed from %s\n", pool);
+    if (libxl_cpupool_cpuremove_cpumap(ctx, poolid, &cpumap)) {
+        fprintf(stderr, "Some cpus may have not or only partially been removed from '%s'.\n", pool);
+        fprintf(stderr, "If a cpu can't be added to another cpupool, add it to '%s' again and retry.\n", pool);
+    }
 
     rc = EXIT_SUCCESS;