[CA-46591] Prevents build_pre from overwriting xen_maxmem.

[CA-46591] Prevents build_pre from overwriting xen_maxmem.

Signed-off-by: Jonathan Knowles <jonathan.knowles@eu.citrix.com>
Previously, during a VM.resume, both of the following functions would overwrite xen_maxmem:

  1. transfer_reservation_to_domain:
     xen_maxmex := reservation
  2. build_pre:
     xen_maxmem := static_max

However, only the first assignment was correct.

All domain creation operations trigger the transfer_reservation_to_domain function. Therefore it is safe to remove the second, faulty assignment.