libxl: introduce XSM relabel on build

libxl: introduce XSM relabel on build

Allow a domain to be built under one security label and run using a
different label.  This can be used to prevent the domain builder or
control domain from having the ability to access a guest domain's memory
via map_foreign_range except during the build process where this is
required.

Example domain configuration snippet:
  seclabel='customer_1:vm_r:nomigrate_t'
  init_seclabel='customer_1:vm_r:nomigrate_t_building'

Note: this does not provide complete protection from a malicious dom0;
mappings created during the build process may persist after the relabel,
and could be used to indirectly access the guest's memory. However, if
dom0 correctly unmaps the domain upon building, a the domU is protected
against dom0 becoming malicious in the future.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>