platform: qemu/virt: Move kernel start address to 4MB alligned

In order to support 32-bit guests move the start address to a 4MB
allignment. As 64-bit kernels have a requirement on being 2MB alligned
let's just make this the default for both 32 and 64 bit kernels.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>