xen: guestcopy: Provide an helper to safely copy string from guest

xen: guestcopy: Provide an helper to safely copy string from guest

Flask code already provides a helper to copy a string from guest. In a later
patch, the new DT hypercalls will need a similar function.

To avoid code duplication, copy the flask helper (flask_copying_string) to
common code:
    - Rename into safe_copy_string_from_guest
    - Add comment to explain the extra +1
    - Return the buffer directly and use the macros provided by
    xen/err.h to return an error code if necessary.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>