vdsm_hooks: Add support for checkimages VDSM hook

Introduce the checkimages VDSM hook - perform consistency
check on a qcow2 format disk image using the QEMU disk
image utility.

v2: remove whitespace noise
    add support for block images

v3: remove more tab noise

VDSM hook alternative to the validateImage SPM command.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=611183

Change-Id: Ib4d88fb46a725d2c525d10ce2fcc808bba5b0ab4
Signed-off-by: Jan Lieskovsky <jlieskov@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/9154
Reviewed-by: Assaf Muller <amuller@redhat.com>
Tested-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>