Add ploop_set_max_delta_size()

Added ploop_set_max_delta_size() function that limit the top delta image size.
Parameter stored as <Max_delta_size>N</Max_delta_size>

Reviewed-by: mpatlasov
Reference: PSBM-26582 Add support for "max delta size" functionality
Reference: ploop@1070751
set_max_delta_size(): use ioctl_device()

We already have the functionality to print an error message
and return an appropriate SYSEXIT_ code when an ioctl
has failed, so let's use it here.

This is an addition to previous commit r1070751 (aka e2685dc)

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
ploop_store_diskdescriptor(): fix an error message

Fix a copy-paste error in commit r1070751 (aka e2685dc).

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
set_max_delta_size(): fix message

...and a comment. This is an addition to commit r1070751 (aka e2685dc).

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
dynload.h: add ploop_set_max_delta_size()

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