Fix vga_screen_dump_blank() PPM generation

vga_screen_dump_blank() was not generating a valid PPM file: the width of the
image made no sense (why it was multiplied by sizeof(uint32_t)?), and there was
only one sample per pixel, instead of three.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
