multifd: Be consistent about using uint64_t

We transmit ram_addr_t always as uint64_t.  Be consistent in its
use (on 64bit system, it is always uint64_t problem is 32bits).

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>