hw/arm/boot: Increase compliance with kernel arm64 boot protocol

hw/arm/boot: Increase compliance with kernel arm64 boot protocol

"The Image must be placed text_offset bytes from a 2MB aligned base
address anywhere in usable system RAM and called there."

For the virt board, we write our startup bootloader at the very
bottom of RAM, so that bit can't be used for the image. To avoid
overlap in case the image requests to be loaded at an offset
smaller than our bootloader, we increment the load offset to the
next 2MB.

This fixes a boot failure for Xen AArch64.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@dornerworks.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Message-id: b8a89518794b4436af0c151ed10de4fa@dornerworks.com
[PMM: Rephrased a comment a bit]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>