iotests: skip 042 with qcow which dosn't support zero sized images

Test 042 is designed to verify operation with zero sized images.
Such images are not supported with qcow (v1), so this test has
always failed.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-6-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
