While doing the XSA-492 work I further noticed an inefficiency with
safe_copy_string_from_guest(). All callers pass PAGE_SIZE as the maximum
buffer size, and with the function adding 1 to append a nul terminator
the resulting allocations are all order-1 ones. Which we'd better avoid.
Require respective callers of hypercalls to nul-terminate the strings
within the buffer supplied. While an ABI change, I think it's an
acceptable one.

1: libxc: drop size parameter from xc_flask_context_to_sid()
2: lib: make safe_copy_string_from_guest() validate input

Jan