Make 'uri' optional for migrate QAPI

'uri' argument should be optional, as 'uri' and 'channels'
arguments are mutally exclusive in nature.

Fixes: 074dbce (migration: New migrate and migrate-incoming argument 'channels')
Signed-off-by: Het Gala <het.gala@nutanix.com>
Link: https://lore.kernel.org/r/20240123064219.40514-1-het.gala@nutanix.com
Signed-off-by: Peter Xu <peterx@redhat.com>

qapi/migration.json

@@ -1757,7 +1757,7 @@
 #
 ##
 { 'command': 'migrate',
-  'data': {'uri': 'str',
+  'data': {'*uri': 'str',
            '*channels': [ 'MigrationChannel' ],
            '*blk': { 'type': 'bool', 'features': [ 'deprecated' ] },
            '*inc': { 'type': 'bool', 'features': [ 'deprecated' ] },