x86/paging: deal with log-dirty stats overflow

x86/paging: deal with log-dirty stats overflow

While the precise values are unlikely of interest once they exceed 4
billion (allowing us to leave alone the domctl struct), we still
shouldn't wrap or truncate the actual values. It is in particular
problematic if the truncated values were zero (causing libxenguest to
skip an iteration altogether) or a very small value (leading to
premature exiting of the pre-copy phase).

Change the internal fields to unsigned long, and suitably saturate for
copying to guest context.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>