xen: arm: probe the kernel to determine the guest type earlier

xen: arm: probe the kernel to determine the guest type earlier

We need to know if the kernel is 32- or 64- bit capable sooner so that we know
what sort of domain we are building when allocating memory to it (so we can
place appropriate limits when allocating RAM to the guest). At the moment
kernel_prepare() decides this but it needs the memory to have already been
allocated.

Therefore split the probing functionality of kernel_prepare() and call it much
earlier. The remainder of kernel_prepare() deals with where to place the
kernel in RAM which can be deferred until kernel_load() so do so.

Document the input and output of kernel_probe() and _load().

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
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