tools/libxl: Plumb a restore boolean into libxl__domain_build_state

tools/libxl: Plumb a restore boolean into libxl__domain_build_state

To fix CPUID handling, libxl__build_pre() is going to have to distinguish
between a brand new VM vs one which is being migrated-in/resumed.

Transcribe dcs->restore_fd into dbs->restore in initiate_domain_create()
only (specifically avoiding the stubdom state in libxl__spawn_stub_dm()).

While tweaking initiate_domain_create(), make a new dbs alias and simplify
later code, and drop the local restore_fd alias as the new dbs->restore is
more intuitive in context.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>