qemu-img: Special post-backing convert handling

qemu-img: Special post-backing convert handling

Currently, qemu-img convert writes zeroes when it reads zeroes.
Sometimes it does not because the target is initialized to zeroes
anyway, so we do not need to overwrite (and thus potentially allocate)
it.  This is never the case for targets with backing files, though.  But
even they may have an area that is initialized to zeroes, and that is
the area past the end of the backing file (if that is shorter than the
overlay).

So if the target format's unallocated blocks are zero and there is a gap
between the target's backing file's end and the target's end, we do not
have to explicitly write zeroes there.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1527898
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20180501165750.19242-2-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>