Handle vga= in -append

Here is a patch I had sent twice to the list 2 years ago.
Hopefuly this time someone will be interested

It adds support for passing vga mode to linux kernel through
vga= option in -append

Signed-off-by: Pascal Terjan <pterjan@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>