Add nomigrate config option to disable migration/restore

Add nomigrate config option to disable migration/restore

The new nomigrate option can be set to non-zero in vm.cfg
(for both hvm and pvm) to disallow a guest from being
migrated or restored.  (Save is still allowed for the purpose
of checkpointing.)  The option persists into a save file
and is also communicated into the hypervisor, the latter
for the purposes of a to-be-added hypercall for communicating
to guests that migration is disallowed (which will be
used initially for userland TSC-related sensing, but may
find other uses).

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>