xen/public: arch-arm: Restrict the visibility of struct vcpu_guest_core_regs

xen/public: arch-arm: Restrict the visibility of struct vcpu_guest_core_regs

Currently, the structure vcpu_guest_core_regs is part of the public API.
This implies that any change in the structure should be backward
compatible.

However, the structure is only needed by the tools and Xen. It is also
not expected to be ever used outside of that context. So we could save us
some headache by only declaring the structure for Xen and tools.

[Stefano: improve comment code style]

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>