xen/arm: Correctly handle non-page aligned pointer in raw_copy_from_guest

xen/arm: Correctly handle non-page aligned pointer in raw_copy_from_guest

The current implementation of raw_copy_guest helper may lead to data corruption
and sometimes Xen crash when the guest virtual address is not aligned to
PAGE_SIZE.

When the total length is higher than a page, the length to read is badly
compute with
    min(len, (unsigned)(PAGE_SIZE - offset))

As the offset is only computed one time per function, if the start address was
not aligned to PAGE_SIZE, we can end up in same iteration:
    - to read accross page boundary => xen crash
    - read the previous page => data corruption

This issue can be resolved by setting offset to 0 at the end of the first
iteration. Indeed, after it, the virtual guest address is always aligned
to PAGE_SIZE.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: George Dunlap <george.dunlap@citrix.com>
[ ijc -- duplicated the comment in the other two functions with this behaviour ]