simplify xenmem_add_to_physmap_batch()

simplify xenmem_add_to_physmap_batch()

There's no need for
- advancing the handles and at the same time using
  __copy_{from,to}_guest_offset(),
- an "out" label,
- local variables "done" and (function scope) "rc".

To better reflect its resulting use also rename the function's "start"
parameter to "extent".

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