Optimize screendump

Optimize screendump

When running kvm-autotest, fputc() is often the second highest (sometimes #1)
function showing up in a profile.  This is due to fputc() locking the file
for every byte written.

Optimize by buffering a line's worth of pixels and writing that out in a
single call.

Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Avi Kivity <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>