fsutils.c: quiet fsck

fsck output can broke tools parser. There is no quiet option
for fsck so just redirect stdout to /dev/null

	# fsck -p /dev/ploop26995p1
	fsck from util-linux-ng 2.17.2
	/dev/ploop26995p1: clean, 12/655360 files, 83119/2620928 blocks

[kir@: errors from fsck will still be shown on stderr]

Reference: PSBM-26738
Reference: ploop@1008262
Added ploop_create_dd()

Added functionality to create DiskDescriptor.xml at custom path

int ploop_create_dd(const char *ddxml, struct ploop_create_param *param)

Reference: PSBM-26581
References: ploop@1031445, ploop@1031841
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
ploop_create_{image,dd}: use ploop_di_add_image ret value

As ploop_di_add_image is always returning proper SYSEXIT_* code,
let's just use it directly, saving a couple lines of code.

This also fixes a few cases of returning SYSEXIT_MALLOC instead
of SYSEXIT_PARAM which ploop_di_add_image() returns sometimes.

Reference: ploop@1031842
Signed-off-by: Kir Kolyshkin <kir@openvz.org>