add a -vga none cli option (Stefano Stabellini)

add a -vga none cli option (Stefano Stabellini)

currently there is no way to fully disable any graphic card device for
the PC architecture.
You can have no graphical output, thanks to -nographic, but you would
have the VGA device connected to your PCI bus anyway.
There is already a convenient -vga option to choose between std, cirrus
and vmware; this patch add the new option "none" to select no graphic
card at all.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6322 c046a42c-6fe2-441c-8c8c-71466251a162