xen: arm: Update arm64 image header

xen: arm: Update arm64 image header

With the Linux kernel commits

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/Documentation/arm64/booting.txt?id=4370eec05a887b0cd4392cd5dc5b2713174745c0

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/Documentation/arm64/booting.txt?id=a2c1d73b94ed49f5fac12e95052d7b140783f800

the arm64 image header changed. While the size of the header isn't changed,
some members have changed their usage.

Update Xen to this updated image header.

The main changes are that the first magic is gone and that there is an
image size, now.

In case we read a size != 0, let's use this image size, now. This does
allow us to check if the kernel Image is larger than the size given in
the device tree, too.

Additionally, add an error message if the magic is not found. This might
be the case with kernel's < 3.12 prior to

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4370eec05a887b0cd4392cd5dc5b2713174745c0

which introduced the second magic.

This is acceptable as the support of Xen for ARM64 in Linux has been added
in Linux 3.11 and the number of boards supported by Linux 3.11 on ARM64 is
very limited: ARM models and X-gene. And for the latter it was an early
support with only the serial and timer upstreamed.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>