xen/arm: raw_copy_to_guest_helper: Rework the prototype and rename it

xen/arm: raw_copy_to_guest_helper: Rework the prototype and rename it

All the helpers within arch/arm/guestcopy.c are doing the same things:
copy data from/to the guest.

At the moment, the logic is duplicated in each helpers making more
difficult to implement new variant.

The first step for the consolidation is to get a common prototype and a
base. For convenience (it is at the beginning of the file!),
raw_copy_to_guest_helper is chosen.

The function is now renamed copy_guest to show it will be a
generic function to copy data from/to the guest. Note that for now, only
copying to guest virtual address is supported. Follow-up patches will
extend the support.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>