xen/arm: Extend copy_to_guest to support copying from guest VA and use it

xen/arm: Extend copy_to_guest to support copying from guest VA and use it

The only differences between copy_to_guest (formerly called
raw_copy_to_guest_helper) and raw_copy_from_guest is:
    - The direction of the memcpy
    - The permission use for translating the address

Extend copy_to_guest to support copying from guest VA by adding using a
bit in the flags to tell the direction of the copy.

Lastly, reimplement raw_copy_from_guest using copy_to_guest.

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