hvm_save_one: return correct data

hvm_save_one: return correct data

It is possible that hvm_sr_handlers[typecode].save does not use all
the provided room.  Also it can use variable sized records.  In both
cases, using:

   instance * hvm_sr_handlers[typecode].size

does not select the correct instance.  Add code to search for the
correct instance.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Release-acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>