qemu: use newer -device video device in qemu commandline

qemu: use newer -device video device in qemu commandline

'-device VGA' maps to '-vga std'
'-device cirrus-vga' maps to '-vga cirrus'
'-device qxl-vga' maps to '-vga qxl'
             (there is also '-device qxl' for secondary devices)
'-device vmware-svga' maps to '-vga vmware'

For qemu(>=1.2), we can use -device to replace -vga for video
device. For the primary video device, the patch tries to use 0x2
slot for matching old qemu. If the 0x2 slot is allocated already,
the addr property could help for using any available slot.
For qemu(< 1.2), we keep using -vga for primary device.