grow_lower_delta(): fix memleak

34 bytes in 1 blocks are definitely lost in loss record 47 of 86
    at 0x4A069EE: malloc (vg_replace_malloc.c:270)
    by 0x3CC5881171: strdup (strdup.c:43)
    by 0x4ECA926: find_delta_names (delta_sysfs.c:83)
    by 0x4EE11B2: grow_lower_delta (merge.c:129)
    by 0x4EE1ADB: merge_image (merge.c:319)
    by 0x4EE3227: ploop_merge_snapshot_by_guid (merge.c:784)
    by 0x4EE812A: do_delete_snapshot (snapshot.c:100)

Reference: ploop@980422
Factor out print_sys_block_ploop()

Since we use the same code in two places, let's not repeat it.

Reference: ploop@983790
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
print_sys_block_ploop(): speed up

It appears that running echo and cat for every file under
/sys/block/ploop* takes much more time than using xargs tail.

Since the files are one-liners we get what we want: list of
file names and their contents, but much much faster:

time find /sys/block/ploop[0-9]*/ -type f -not -name '*event' -exec echo {} \; -exec cat {} \; > /dev/null 2>&1

real	0m0.635s
user	0m0.013s
sys	0m0.101s

time find /sys/block/ploop[0-9]*/ -type f -not -name '*event' | xargs tail | grep -v '^$' > /dev/null 2>&1

real	0m0.010s
user	0m0.002s
sys	0m0.007s

Note we have to add grep -v '^$' to filter out empty lines that tail
adds after each file.

Reference: ploop@983791
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
print_sys_block_ploop(): don't show ve_device_add

This file can't be read and we don't want an error message about it.

Reference: ploop@983792
Signed-off-by: Kir Kolyshkin <kir@openvz.org>