Make 'uri' optional for migrate QAPI

Make 'uri' optional for migrate QAPI

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

Fixes: 074dbce5fcce (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>
(cherry picked from commit 57fd4b4e10756448acd6c90ce041ba8dc9313efc)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>