qemu-io: Use correct range limitations

create_iovec() has a comment lamenting the lack of SIZE_T_MAX. Since
there actually is a SIZE_MAX, use it.

Two places use INT_MAX for checking the upper bound of a sector count
that is used as an argument for a blk_*() function (blk_discard() and
blk_write_compressed(), respectively). BDRV_REQUEST_MAX_SECTORS should
be used instead.

And finally, do_co_pwrite_zeroes() used to similarly check that the
sector count does not exceed INT_MAX. However, this function is now
backed by blk_co_pwrite_zeroes() which takes bytes as an argument
instead of sectors. Therefore, it should be the byte count that does not
exceed INT_MAX, not the sector count.

Signed-off-by: Max Reitz <mreitz@redhat.com>
