mount: Read multiple mntns images (if any) on restore

read_mnt_ns_img() is splited on two parts. The first part enumirates all
namespaces and call the second part, which read a proper image file.

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