Fix two XBZRLE corruption issues

Push zero'd pages into the XBZRLE cache
    A page that was cached by XBZRLE, zero'd and then XBZRLE'd again
    was being compared against a stale cache value

Don't use 'qemu_put_buffer_async' to put pages from the XBZRLE cache
    Since the cache might change before the data hits the wire

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
