CA-146652: xen-ringwatch generates too many false positives in crit.log

The problem arises because tapdisk3 updates a VBD's ring stats file every 5
seconds, while xen-ringwatch expects pending requests to have been completed in
1 second. This results in notifications of unserviced requests being generated
in crit.log. To address the problem we need to guarantee that 'stats' is
updated at least once per xen-ringwatch call. Thus, tapdisk3's 'stats' update
interval is lowered to 3 seconds and xen-ringwatch's window is raised to 4
seconds (add mercurial ref).

Signed-off-by: Kostas Ladopoulos <Konstantinos.Ladopoulos@citrix.com>