restore: add an option for changing a root file system (v2)

The option is -r|--pivot-root and an argument is a path to new root.
A root task will make pivot_root. LXC CT does that, so we need that
for restoring.

v2: s/pivot-root/root/

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/crtools.h

@@ -87,6 +87,7 @@ struct cr_options {
 	unsigned int		namespaces_flags;
 	bool			log_file_per_pid;
 	char			*output;
+	char			*root;
 };
 
 extern struct cr_options opts;