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:  <wangxiaolong@ucloud.cn>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 50824995)
Conflicts:
	block/io.c

* applied manually to avoid dependency on 61007b31
* squashed in 6e82e4bc bdrv_reset_dirty() is static in
  2.3.0 and becomes unused as of this patch
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>