We need to know this to insert holes. Currently xfer->parent isn't
initialized for remote sessions. In most cases it has a non-zero value,
so generate_iovs() is called with has_parent = true.

bash test/zdtm.sh -p -P -i 3 ns/transition/fork
(00.000106) Error (sysctl.c:194): Can't open sysctl net/ipv4/tcp_wmem: No such file or directory
(00.017048)    420: Error (image.c:231): Unable to open pagemap-420.img: No such file or directory
(00.017065)    420: Error (image.c:231): Unable to open pages-420.img: No such file or directory
(00.017090)    420: Error (page-read.c:73): No parent for snapshot pagemap
(00.017290)     86: Error (cr-restore.c:1185): 420 exited, status=1
(00.017317) Error (cr-restore.c:1831): Restoring FAILED.

v2: add a new command to open a page server. It's required to save
backward compatibility. If someone tries to use an old version of
page server, he will get an error.

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