migration: cpr-exec-command parameter

migration: cpr-exec-command parameter

Create the cpr-exec-command migration parameter, defined as a list of
strings.  It will be used for cpr-exec migration mode in a subsequent
patch, and contains forward references to cpr-exec mode in the qapi
doc.

No functional change, except that cpr-exec-command is shown by the
'info migrate' command.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Link: https://lore.kernel.org/r/1759332851-370353-5-git-send-email-steven.sistare@oracle.com
Signed-off-by: Peter Xu <peterx@redhat.com>