vzctl start: umount ploop image on CT start

If ploop image is mounted (but fs is not), CT fails to start:

Adding delta dev=/dev/ploop1 img=/srv/vz/private/400/root.hdd/root.hdd (rw)

Starting container ...
Error in ploop_mount_image (ploop.c:1251): Image /srv/vz/private/400/root.hdd/root.hdd already mounted to /dev/ploop1
Failed to mount image: Error in ploop_mount_image (ploop.c:1251): Image /srv/vz/private/400/root.hdd/root.hdd already mounted to /dev/ploop1 [21]

Use ploop_find_dev() to check if an image is mounted, and unmount it.

Reference: PSBM-11340
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
4 files changed