monitor: Improve and document client_migrate_info protocol error

Protocol must be spice, vnc isn't implemented.  Fix up documentation.

Attempts to use vnc or any other unknown protocol yield the misleading
error message "Invalid parameter 'protocol'".  Improve it to
"Parameter 'protocol' expects spice".

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by. Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
