migration: hmp: dump globals

Now we have some globals that can be configured for migration. Dump them
in HMP info migration for better debugging.

(we can also use this to monitor whether COMPAT fields are applied
correctly on compatible machines)

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1498536619-14548-11-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>