hvm/hpet: Init comparator64 like comparator.

hvm/hpet: Init comparator64 like comparator.

The software-developers-hpet-spec-1-0a.pdf says that the comparator
starts as all 1's.  Also make the hidden register comparator64 the same.

Since only the hidden register comparator64 is used by hpet_save, it
needs to start out with the right value.

A disabled hpet (like when a guest is starting), should start with
the value the spec says.  Both the guest (via reading the
comparator) and an administrator using xen-hvmctx, will see all 0's
not all 1's.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>