block: Fix dirty bitmap in bdrv_co_discard

block: Fix dirty bitmap in bdrv_co_discard

Unsetting dirty globally with discard is not very correct. The discard may zero
out sectors (depending on can_write_zeroes_with_unmap), we should replicate
this change to destination side to make sure that the guest sees the same data.

Calling bdrv_reset_dirty also troubles mirror job because the hbitmap iterator
doesn't expect unsetting of bits after current position.

So let's do it the opposite way which fixes both problems: set the dirty bits
if we are to discard it.

Reported-by: [email protected]
Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>