service: Keep images in case restore failed

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Select FS hauler in OVZ driver

1. Python doesn't help with getting filesystem name :(
2. OVZ can exist on shared or local filesystems, for
   now just check names explicitly, need better selection
   in the future.
3. ext3/4 fs hauler is empty, this migration would just
   fail

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Add basic calls for FS migration

The start_migration should perform first (expected to be the
longest) FS sync.

The next_iter() is notification about regular migration process
went on the 2nd iteration to copy more memory.

The stop_migration() is to perform final FS sync and stop it.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs_subtree: Specify what directory to rsync

It's the CT's private dir one.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>