iotests: Add test 179 to cover write zeroes with unmap

iotests: Add test 179 to cover write zeroes with unmap

No tests were covering write zeroes with unmap.  Additionally,
I needed to prove that my previous patches for correct status
reporting and write zeroes optimizations actually had an impact.

The test works for cluster_size between 8k and 2M (for smaller
sizes, it fails because our allocation patterns are not contiguous
with small clusters - in part, the largest consecutive allocation
we tend to get is often bounded by the size covered by one L2
table).

Note that testing for zero clusters is tricky: 'qemu-io map'
reports whether data comes from the current layer of the image
(useful for sniffing out which regions of the file have
QCOW_OFLAG_ZERO) - but doesn't show which clusters have mappings;
while 'qemu-img map' sees "zero":true for both unallocated and
zero clusters for any qcow2 with no backing layer (so less useful
at detecting true zero clusters), but reliably shows mappings.
So we have to rely on both queries side-by-side at each point of
the test.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170507000552.20847-10-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>