qcow2: Add iotest for an empty refcount table

This patch adds a simple iotest in which we try to write to an image
with an empty refcount table (i.e. with all entries 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>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 7e48b0e2ae1a0a18e0ee303b3045f130feec0474.1509718618.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
