Note that resetting bits in the dirty bitmap is done _before_ actually
processing the request.  Writes, instead, set bits after the request
is completed.

This way, when there are concurrent write and discard requests, the
outcome will always be that the blocks are marked dirty.  This scenario
should never happen, but it is safer to do it this way.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>