qemu-img convert can assume "that sectors which are unallocated in the
input image are present in both the output's and input's base images".

However it is only doing this if the output image returns true for
bdrv_has_zero_init().  Testing bdrv_has_zero_init() does not make much
sense if the output image is copy-on-write, because a copy-on-write
image is never initialized to zero (it is initialized to the content
of the backing file).

There is nothing here that makes has_zero_init images special.  The
input and output must be equal for the operation to make sense, and
that's it.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>