vzmigrate: don't run vzfsync if there is no need

In case of ploop-based container migration, if there are no snapshots,
there is no need to call vzfsync as there are no images that rsync
copied (i.e. there is only a top delta, which is transferred by ploop
copy). Therefore, vzfsync is called with zero file arguments, giving
this error:

	   Calling vzfsync on ploop deltas
	Usage: vzfsync {-s|-d} [-n] file [file ...]
	       -s, --sync		do fsync()
	       -d, --datasync		do fdatasync()
	       -n, --dontneed		do fadvise(DONTNEED)

To fix, let's get the list of deltas to sync first, and only run vzfsync
if the list is not empty.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>