qemu-img convert: Rewrite copying logic

The implementation of qemu-img convert is (a) messy, (b) buggy, and
(c) less efficient than possible. The changes required to beat some
sense into it are massive enough that incremental changes would only
make my and the reviewers' life harder. So throw it away and reimplement
it from scratch.

Let me give some examples what I mean by messy, buggy and inefficient:

(a) The copying logic of qemu-img convert has two separate branches for
    compressed and normal target images, which roughly do the same -
    except for a little code that handles actual differences between
    compressed and uncompressed images, and much more code that
    implements just a different set of optimisations and bugs. This is
    unnecessary code duplication, and makes the code for compressed
    output (unsurprisingly) suffer from bitrot.

    The code for uncompressed ouput is run twice to count the the total
    length for the progress bar. In the first run it just takes a
    shortcut and runs only half the loop, and when it's done, it toggles
    a boolean, jumps out of the loop with a backwards goto and starts
    over. Works, but pretty is something different.

(b) Converting while keeping a backing file (-B option) is broken in
    several ways. This includes not writing to the image file if the
    input has zero clusters or data filled with zeros (ignoring that the
    backing file will be visible instead).

    It also doesn't correctly limit every iteration of the copy loop to
    sectors of the same status so that too many sectors may be copied to
    in the target image. For -B this gives an unexpected result, for
    other images it just does more work than necessary.

    Conversion with a compressed target completely ignores any target
    backing file.

(c) qemu-img convert skips reading and writing an area if it knows from
    metadata that copying isn't needed (except for the bug mentioned
    above that ignores a status change in some cases). It does, however,
    read from the source even if it knows that it will read zeros, and
    then search for non-zero bytes in the read buffer, if it's possible
    that a write might be needed.

This reimplementation of the copying core reorganises the code to remove
the duplication and have a much more obvious code flow, by essentially
splitting the copy iteration loop into three parts:

1. Find the number of contiguous sectors of the same status at the
   current offset (This can also be called in a separate loop before the
   copying loop in order to determine the total sectors for the progress
   bar.)

2. Read sectors. If the status implies that there is no data there to
   read (zero or unallocated cluster), don't do anything.

3. Write sectors depending on the status. If it's data, write it. If
   we want the backing file to be visible (with -B), don't write it. If
   it's zeroed, skip it if you can, otherwise use bdrv_write_zeroes() to
   optimise the write at least where possible.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
