qcow2: Add iotest for an image with header.refcount_table_offset == 0

This patch adds a simple iotest in which we try to write to an image
with the refcount table offset set to 0.

This scenario was already handled by the existing consistency checks,
but we add an explicit test case for completeness.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: feeceada92486bb8790b90f303fc9fe82a27391a.1509718618.git.berto@igalia.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
