block: allow commit to unmap zero blocks

block: allow commit to unmap zero blocks

Non-active block commits do not discard blocks only containing zeros,
causing images to lose sparseness after the commit. This commit fixes
that by writing zero blocks using blk_co_pwrite_zeroes rather than
writing them out as any other arbitrary data.

Signed-off-by: Vincent Vanlaer <libvirt-e6954efa@volkihar.be>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20241026163010.2865002-5-libvirt-e6954efa@volkihar.be>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>