screendump: replace FILE with QIOChannel and fix close()/qemu_close()

screendump: replace FILE with QIOChannel and fix close()/qemu_close()

The file opened for ppm_save() may be a /dev/fdset, in which case a
dup fd is added to the fdset. It should be removed by calling
qemu_close(), instead of the implicit close() on fclose().

I don't see a convenient way to solve that with stdio streams, so I
switched the code to QIOChannel which uses qemu_close().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>