The old code treats bits as bytes when calculating host memory usage.
Change it to be consistent with allocation logic in pixman library.

Signed-off-by: Tao Wu <lepton@google.com>
Message-Id: <20171109181741.31318-1-lepton@google.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>